Above all questions about how to do it, what restrictions, which versions of games, about regulations and stuff like responsibility and accountability, and so on stand big questions, and i'd say it's the most important ones (for those companies at least):
What about the money?
Does MS get money from Steam and Epic if their stores are on Xbox consoles?
How much money do they get?
Do they get a "general fee" or a part of every game sale?
Do Steam and Epic actually want to share revenue with MS?
Remember, the Epic vs Apple drama went on because Epic didn't want to pay the regular fees Apple charges for this Appstore stuff.
So why would they feel okay with paying a share to MS?
Would MS gain anything, outside of short-lived internet goodwill, from allowing Steam and Epic having their stores on their hardware for free?