Q1) I think Doom is going to be a much bigger problem than people think outside the Meta, he is tricky with punishing yellow and black matches and running with Okoye gives you healing galore. You'll never use his yellow so it won't hurt a half health Thor team but will punish an enemy Okoye one. I would be tempted to softcap until you get Thor but then I don't have any 5* champs to compare how that might work. Adding Hela to that team to punish Thor/Okoye doubly sounds like a nice idea in theory also but I would look to others opinions on that as to how that would work out for you.
Q2) Well presumably once Thor joins Okoye you will never look at any of these guys again! In the meantime, completely my badly formed ill judged opinion and feel free to disregard:
Q3) Shuri and her fortification is a nice power, so maybe 4BP so you can run them together? Thanos, Magik and Domino are all solid, my favourite is probably Magik though. The Devs seemed to realise that Talos is gutter level trash tier and didn't even bother making people jump thru hoops for SHIELD Training but I guess he might pop up...I would actually myself BH Sabretooth because he looks extremely kickbottom but obviously that will be a longer term plan.