That is indeed one of the possible options (at least in the current plan), so swell away. 
The sword will accept any Guen with a high enough dark magic stat.
Well… how would you feel if you best friend got romantically/sexually involved with your child (who is 20+ years younger?) Morgana will never completely not think it’s weird, but her level of acceptance will probably vary depending on where she is emotionally at the time, and whether or not Guen is cheating on her with her own son.
Yeah. 
Probably.
It can (or it might not). It will depend on Morgana’s feelings about a lot of things, and the strength of their friendship. I’m really looking forward to juggling all the different factors that might make Morgana side either with Mordred or with Arthur/Lancelot in the final conflict.
Morgana is somewhat loyal to Britain (mostly in the sense that she’s loyal to Avalon, and Avalon is part of Britain), but at the moment, her loyalty is primarily to Arthur, because he’s one of a very few people who treat her like a human being.
@shirette Nice to see you again! 