A:AnswerCause it already has a built in back plate for an am4 cpu cooler, you should have an am4 bracket with your aio from corsair, if not your aio is too old.
A:AnswerNo. This is for AM4 cpus. A10-7800 is a FM2+ cpu.
You are not able to find a new FM2+ motherboard from any big resellers, since that platform is more than 7 years old.
A:AnswerYes, the case and power supply of the Aegis ZS 3CQ can support that CPU, but there is no real gain in swapping a motherboard other than for I.O purposes.