Kaja
1 and 3) Whether I have a belt on or not I practically always wear boxer briefs. I wear them because of my tendency to rub my thighs. The exception is sometimes on hot summer days when I'm sitting at home. That's when I'm in just my belt.
2) I can wear whatever underwear I want - my mum doesn't interfere.
4) I always wear a bra, but this is not a requirement of mum, just a practical matter - my breasts need to have support.