Yeah I think running time is gonna be a major factor. If it's full length I can see it working, but I wouln't be happy if it is like two 40 minute episodes.
Anime movies are only that short if they're part of a double or triple feature, these seem to be standalone movies and I expect them to be full length.
We're getting Stars unless these movies are a total flop. You all worry too much in my opinion, Toei wouldn't switch to theatrical releases and announce two at once if they didn't have faith in Sailor Moon. If they wanted to wrap it up and throw fans a bone at the same time they could also do a single tv special.
Eh, the first season (two seasons) tried to adapt the manga 1:1 (well, with few exceptions), and the pacing was awful: some episodes were rushed to fit all the action, others were mostly an exposition dump. A long two-part movie might be a better fit since it allows you to better pace things around instead of following the manga act by act.