We see people in civilian clothes and in costumes for the holodeck all the time. This does not seem to be a concern. You dress for what ever activity you're engaged in. Be it exercise, repair or sitting at desk.
Scants aren't exactly tight skirts. Not sure if they restrict mobility. Though maybe some pleats are needed.
Probably not. The ship seems to be set for whatever normal is for the majority of the crew. Individual quarters seem to have individual settings.
We see them in those other outfits when not on duty. Those uniforms seem like raising your arms would reveal underwear and they seem awfully tight to have full range of motion in arms.
Most decks are human range of comfortable temperature but you don't think that they ever transport diplomats from cold planets? You think they make those diplomats travel through a searing heatscape to talk to each other?