Oct. 15, 2017, 9:07 p.m.
1- You're getting GTAV and GTAOnline mixed up. GTA V is the solo campaign with Michael, Trevor & co. GTA Online is its always online counterpart which uses the same setting, map and vehicles than V, with exclusive free DLC bonus content (which you can easily add to SP with mods). Rockstar actually supports the modding community for the single player game, like we saw it with the OpenIV fuckup 2 months ago; however you will not be able to connect to GTA Online with your modded V stuff (you will have to remove scripthook from your game folder for instance). Forced log in on GTA Online with a modded game can indeed lead to a banned account. Never tried it though.
2- I dont really get your question. In SP you get to play with all 3 main charachters. Saving with one will save all 3. You cannot play with them on Online though; you'll have to create your own avatar.