sure, check it: uConsole-Sleep v1.3
also people put cm5 and downlcock it to have similar to cm4 battery life, but better perfomance.
ehm, you can’t even normally use modern browser on cm4. (on core2duo you can’t eihter)
you can check internet for comparison of CPUs performance.
no, but there non-official board that allow you to replace 18650x2 with lion square battery, it will have more capacity if you are willing to do it.
with 2x18650 you should have ~5 hours. (±2h)
did you mean radxa? it works, but no audio and wifi so you need to use usb dongle.