1.) The aquaero comes with four foil temperature sensors but I recommend to get another temperature sensor like
this one in addition. It allows to measure the water temperature which is pretty much the most important one since it is also a perfect base to adjust the speed of the fans.
2.) The pump can be connected to the aquaero via aquabus but since you also want to add a reservoir with a fill level sensor (also aquabus), you will need a
aquabus y-cable.
A) For the initial setup you will need the USB connection but afterwards you can only connect the pump and fill level sensor via aquabus. For the fill level sensor you will need a
4-pin aquabus cable when using it without USB. Normally the sensor gets its power through USB but since you do not want to connect this the power must be provided through the aquabus. For the pump it does not matter if you use the supplied 3-pin cable or an optional 4-pin cable because it must be powered through the PSU anyway.
B) Yes, for the aquaero and the pump.
C) It offers no real advantage beside the fact that it will just provide one more sensor port. This port is mainly interesting for those who use the pump without an aquaero.
D) Pretty much the same as point C. It is interesting when used without an aquaero.
3.) If four fan channels are enough, then you do not need a poweradjust. Of course you can use splitters or y-cables to connect a bunch of fans to a single fan channel.
4.) The aqualis XT has an integrated fill level sensor which - surprise, surprise - monitors the fill level

This value can be also transfered to the aquaero via aquabus to set up an alarm if the level comes down to a specific value.