Welcome to the forum. I will try to answer your questions as best I can.

1 – You can’t connect USB ports on multiple Aquacomputer (AC) devices to each other. Each one needs to go to its own USB port to the mobo. The solution is to install an internal USB 2.0 hub that is Sata or Molex powered. Connect the hub to one of your mobo USB2.0 ports then connect the AC device’s USB ports to the hub. AC sells the HUBBY7 which works very well for this purpose. The NZXT AC-IUSBH-M3 USB2.0 Hub is a tried and true alternative, which, unlike the Hubby7, is in stock at multiple online sites. I just bought one HERE because I could not find a HUbby7 in stock in the US. Keep away from cheap USB2.0 hubs on Amazon that aren’t Sata or Molex powered.

2 – With your devices, yes you can ignore aquabus. Aquabus is a legacy serial control protocol that has limited bandwidth. It is mainly used to connect devices to an Aquaero. The “Next” series of devices rely on USB communication with the Aquasuite (AQS) software. Some devices, like the Octo, still have an aquabus port but this is for connecting the Octo to an Aquaero, which will make the Octo a slave of the Aquaero controller.
The aquabus port on the D5 Next is used to connect the pump to an Aquaero. The manual says it can also be used as an RGBpx port (not at the same time). I am not clear on how this works because AC reps have said many times that aquabus has limited bandwidth and cannot carry the amount of data required by RGBpx. Maybe someone else can chime in here and clarify this. The aquabus port on the High Flow Next is for connection to an Aquaero. It has no other function.

3. Since you have a High Flow Next, which will be connected to the mobo via the USB2.0 hub, there is no need to use the flow sensor headers on the Octo or the D5 Next. There actually is a reason to connect the High Flow next Signal output to the D5 Next Fan/Flow header (with cable #53294) but it’s kind of complicated to explain. It has to do with the D5 Next virtual flow sensor being used as the data source for the pump’s Alarm function. If you want to know the details of this let me know and I will try to explain it.

4 - I don’t own an Aquaero so I don’t know a lot about it. It’s an older controller and I don’t think they are available anymore. You can still find them in stock here and there. AC said they are not making them now because some of the component prices have skyrocketed and some have 18-month lead times. They may resume manufacturing at some point in the future, or maybe they will release a new model.
The Aquaero does do some things that the Octo can’t, starting with being able to drive DC and PWM fans. The CaliTemp sensors can only be used with an Aquaero. The Aquaero also has a relay output and an Aquastream Bridge connector for the Aquastream pumps. The Aquaero is a complex device. If you are operating Linux it may be a consideration because the Next devices with USB require the Aquasuite service to be running which requires Windows. If you are running Windows, it is doubtful that an Aquaero would work better unless you have some unusual requirements.

5 - Aquasuite cannot control your mobo LEDs. It can control DRGB LEDs in fans, LED strips, etc. They have adapters to go to and from their RGBpx connectors (Molex Picoblade) to the standard (and horrible) DRGB connectors. Each RGBpx port can drive 90 DRGB LEDs.

6 – Yes. The Leakshield sensors will report the High Flow Next flow rate and the D5 Next Pump speed. All the devices have assignable virtual sensors as well.

Note - be sure to check out the Playground area of Aquasuite. This is where the real power lies. You can combine and process sensor data here and build virtual sensors that are based on real sensors. A very basic example is subtracting ambient temp from coolant temp, then using the difference between these temps (delta-t) as the control source for your fans. You could have the fan PWM ports track delta-t but also kick up to a higher speed if your CPU and/or GPU exceed a threshold temp for a period of time. If you have a temp sensor at the inlet and outlet of a rad, and you know the flow rate, you can calculate how many watts of heat the rad is dissipating. The possibilities are endless.

As the leakshield can draw up to 500 ma, it should be connected to a usb port on the mobo, by itself.
Do not connect the leakshield to a usb splitter

Quoted from "InfoSeeker"

As the leakshield can draw up to 500 ma, it should be connected to a usb port on the mobo, by itself.
Do not connect the leakshield to a usb splitter

This is a good point but I thought it was OK to connect the Leakshield to a Sata powered USB hub. My Leakshield is plugged into a Hubby7 port and works fine. The Hubby7 is in Sata power mode. Is this a problem?

i believe it may be if you want the leakshield to operate while the PC is off ?.. some suspend states allow the motherboard USB headers to still be powered even when off if i'm not mistaken. That wouldn't work with SATA power. but it's probably a niche use of the leakshield.
I may be completely wrong but i believe i red about that somewhere here.

Basically, Aquabus is a legacy feature by now. USB control is the norm with all new products. Aquabus still has its uses for people not using windows, but it's a very special use case.
Through Aquasuite, you can use any sensor to control any device. The Aquasuite service takes care of everything even without physical interconnexions.
All devices have Software sensors where you can add sensors from another device. For example, if you want to use the temperature sensor of the High flow NEXT on the octo, just open the sensors page of t he Octo, pick a software sensor and add the HFNext temperature sensor in it. then that sensor is available for the Octo to use.

Or of course, you can do your Playground virtual sensors, using combinations of various hardware sensors from various devices.
You then add that virtual sensor to any device on one of its free software sensors to be used for control.

And yes, a little video series explaining how to use the software would be cool at least to explain the basics, the concept of software sensor and maybe a few basic examples with the playground.

If you run out of USB2.0 headers on your mobo, just get a Hubby7 or an NZXT AC-IUSBH-M3. They are both Sata powered and quality products (which is surprising from NZXT). The only downside is they are not going to stay powered when the computer is off, so plan your port loading accordingly.

Regarding Aquasuite's documentation, their manuals are a good starting point but they do not cover everything. While they could do a better job with their documentation, it would be impossible to document everything that can be done in the Playground. There are several good sources for additional information, starting with this forum.
Hardwareluxxe also has an Aquasuite forum. It’s in German but your browser should be able to translate it. There is also a Discord channel called Aquacomputer Unofficial but sadly it is not very active. Finally there are a few instructional videos from Kaybee Tech on Youtube. He has a Beginner's Guide and several videos about doing some things in the Playground. This should get you started.

I also recommend that you consider importing data from HWINFO into Aquasuite via the Shared Memory Support interface. Aquasuite does directly poll sensors but not as many as HWINFO and its sensor polling it not updated as often as HWINFO. I recently posted about the advantages of doing this HERE. Aquasuite can also import data from AIDA but I don't know how that compares because I import sensor data from HWINFO.

Quoted from "Remayz"

i believe it may be if you want the leakshield to operate while the PC is off ?.. some suspend states allow the motherboard USB headers to still be powered even when off if i'm not mistaken. That wouldn't work with SATA power. but it's probably a niche use of the leakshield.
I may be completely wrong but i believe i red about that somewhere here.

My LKS is still in a test loop so it currently does not need to stay powered up when the computer is off. When the loop goes in the case, I will want to keep the LkS powered on all the time, so I will have to move its USB from the Hubby7 to a port that stays powered. I am a bit concerned about how much current the USB2.0 headers on my Asus Crosshair VIII Dark Hero mobo can provide.

There are 2 headers (USB1213 and USB_E56). The mobo manual does not provide any current specs but in THIS POST forum member DaL iu claimed that the USB_E56 port has half the current capacity of USB1213. FWIW, I dug into this and think he was confused. The Crosshair VII Hero’s 2nd USB2.0 header (USB15) had only 1 set of data pins wired but on the Crosshair VIII Hero and Dark Hero, both USB2.0 headers have both sets of data pins wired. This also has nothing to do with how much current they can provide. I think the 2 headers on my Dark Hero are functionally the same but I still don’t know how much current they can source. The USB2.0 spec says 5V @ 500ma so hopefully the Asus USB2.0 headers are both spec-compliant.

Currently, I am more concerned about pulling too much current through the Hubby7 port. The manual says 1000ma total, split 500ma for ports 1 to 3 and 500ma for ports 4 to 7. My LKS is plugged into port 3, Octo in port 1, and D5 Next in port 2. Octo power is Molex, D5 Next power is Sata, but I don’t know how much current they pull through their USB ports. I did some digging and found THIS WEBSITE which says, "High Speed (480Mbits/s) mode uses a 17.78mA constant current for signalling to reduce noise." If this is correct, then the Octo and D5 Next USB ports are pulling 35.56ma total leaving only 14.44ma for the LKS. It has been working this way for months but I think I should move the Octo and D5 Next to different ports and leave ports 1 and 2 open so the LKS has the whole 500ma available.

This thread should help you.

Guides and Videos that help beginners
Check, double check and recheck your order to make sure you have everything you need and then some, especially if you are ordering directly from Aquacomputer. It can get tricky. Get extra cables, reservoir filters, seals etc. as a backup since they are cheap. That is what I did because I can't find a lot of this stuff locally in the US. I noticed they got a lot of stuff back in stock.

"cough" leakshield membranes