1) You should be able to, I don't see why not.
2) I'm not sure about floors, haven't tried it, but you can create pathways and floors using the terrain paint tool, if you don't mind
stone ones.
3) I don't use CC, can't help you with that.
4) That depends on what kind of things give you headaches. I can't think of any game destroying things at the moment.
5) So, you killed a Sim and you have a tombstone and an urn? Pretty sure the tombstone is supposed to turn into the urn if
you bring it inside. If you don't want to deal with ghosts then you can release the spirit to the netherworld and sell/delete the
urn/tombstone. No, there is no graveyard in Sims 4 but you can place graves on community lots.
I'm not much of a builder so I can't help much with a lot of those things, but there are building guides on this site and a building thread on this forum. Also, a Youtuber who goes by the name, TheSimSupply, has a lot of building tutorials on his channel that are really easy to follow, even for building n00bs like myself, he covers everything, so check them out.