Bib Shorts question

I'm with Dingaling on this one. I always have a shower before riding or at the very least give everything a thorough antibacterial wet wipe if Mr Hankey makes a post-shower/pre-ride appearance.

In terms of re-using shorts, no. Don't be tempted to do this. Get some cheap secondhand shorts or some DHB specials and they'll be more than enough for a short turbo session. 

You can get away with it... you'll soon develop a cast iron gooch and what's a bit of ischial tuberosite bruising to superhero who's putting in the miles in his lunchhour

I don't know how sweaty you get while turbo training, but I would have said wearing underwear won't be enough to prevent the need to wash your shorts and therefore you will have just created more washing, not less.

FWIW I often wear underwear under my bibtights in winter for extra warmth and don't find I have any problems with chafing etc if that's what you're worried about. Although go for proper sports underwear, not M&S cotton pants!

Yes, you can wear underwear beneath your shorts.
I sometimes wear underwear under my winter bib shorts, without any issues.
Just try it yourself and see what you think. There's a 50/50 chance you'll like it or not.

Cycling "rules" are advisory in my opinion.

I don't bother with any sort of cycling shorts or pad for a 45 minute ride. Just do it in some comfortable clothing.

Do not wear anything under your cycling shorts.

I would buy some extra shorts. Cheap waist shorts from Decathlon, Lusso, DHB etc will be fine for 45 minutes, possibly a lot longer. You should wash them frequently. Wash your undercarriage after every session.