A) 1. Recovery: Yes both are essentially the same but download MicroG one just for the sake of it.
TWRP is not required for flashing MicroG Lineage OS. Linease OS recovery is sufficient. Look at your phones XDA forums and installation guided there will be some Roms to check which recovery to use for flashing anything with android 11 and use that.
Make a backup of everything using a custom recovery of your choice and then start wiping your phone.
B) 1. Nearby Share is not available on MicroG as Nearby Share is directly implemented in Play Services. Would be useful on MicroG but probably is not the priority rn.
I don't know where you are from. If your from India, there are multiple alternatives like PayTM, PhonePe, Amazon Pay and more.
But we have 2 great FOSS alternatives. Snapdrop and TrebleShot. You cans share files without even installing the app for snapdrop directly over the internet if you want an app you can find one on F-Droid or check out TrebleShot which works very similar to NearbyShare but needs an App.
C) 1. Depends on your device. On my OP5T, Its fairly easy. Flash your favorite custom recovery, wipe the partitions and install LOS4Mg. 2. Adb sideload shouldn't stop at 47%, it must be some sort of fault from your side. Please check the your device screen to see the error message its outputting.
For some reason I could never get KDE Connect to work. Maybe my work and home wifi networks are just too complex, but I've never gotten it working.
Trebleshot is a FOSS app that doesn't even depend on the other device having the app, has multiple modes of transfer, offline transfer. If you just want to occasionally drop things to random devices without needing any other setup from them, the is a fantastic way to do it
TrebleShot (Send and receive files over available connections) - https://f-droid.org/packages/com.genonbeta.TrebleShot
What you need it for?
There is Trebleshot which works for Android to Android, and there is a Windows, Linux and MacOS programs under development. It would require both devices to have the app downloaded.
There is ShareViaHttp which is android to any device with a browser. There are some limitations as if you share multiple files, it would be in the form of a zip file.