1 yes if no other drives dvd or removable are plugged in then D should be the 1st letter it adds.
2 Dolphin Citra PPSSPP and Desmume are all stand alone emulators that I use instead of the RA cores for whatever reason. You can always try those cores along with the stand alones and see which you actually prefer. Demul has no core but I do recommend it over any of the RA cores for Dreamcast.
3 MAME and things like FBA have years next to them for older cores the latest up to date ones are just the name which ones you want to use will depend on your ROM set. Some cores will depend upon your system specs which emulators will be best to use.
4 Updating the emulator or LB shouldn't effect anything as long as your paths don't change. Definitely updating LB should never effect your emulators working.
5 Demul needs specific setup in LB there are tons of posts and tutorial that explain everything you need to know
6 Yes some need bios and you do need to find them on your own even certain systems in RA will require bios files to be put in the systems folder of RA
7 for PCSX2 you will find that you can't run all games on the same settings some will need special settings.
I see Neil already answered as I was typing this so hopefully I don't bore you with a bunch of redundant information