1) I've noticed similar issues with sneak attacks. Would be nice if it confirmed whether or not sneak attack would apply before you do it. Ideally it would just apply sneak attack automatically to any eligible attack, as a passive ability rather than an active one, which would allow it to combo better with things like special arrows.
2) There's an icon on the left that appears that breaks down the percentage chances, including advantage/disadvantage. Doesn't give a lot more insight, but its something.
3) I haven't had this from opportunity attacks, but have had similar issues where I've had characters randomly lose actions.
4) That mephit encounter is terrible. Only reason I survived was because the mephit ai bugged out and they wouldn't actually do anything on their turns.
5) I've had that same issue with Ethel, I think its related to insisting on asking about Mayrina in the previous dialogue with her, as I didn't have it the times I ignored Mayrina and took her deal. Good idea for the workaround, didn't think to check if she was their invisibly.
Also on the note of the Ethel fight, the one time it worked for me I took her second deal to let her leave in return for more 'power'. However I never received the promised attribute point...