Bar end shifters and down tube shifters share the same mounting interface, and there's a bracket(paul's thumbies) that lets you turn either into a thumb shifter. There's another bracket (Jag-something) that lets you turn a DT shifter into a BE shifter. Don't know if something that started out as a thumb shifter would be compatible or not.
Bar diameters, you'll have to check. There are plenty of bars that aren't straight, but still follow the MTB standard for grip area diameter, Brahma bars, trekking/butterfly bars, Nitto North Road etc. Nitto moustache OTOH is posted as "Road brake levers".
BBs are a so-so question. Cartridge BBs aren't generally seen as serviceable, but apart from the very rare lemon, they tend to do their jobs for years and years and miles and miles w/o any fuss - particularly the square taper ones. Unless you're planning to cross Mongolia or the Australian outback, a modest amount of attention will enable you to to have the BB replaced before a recently noticed bearing issue will turn into a critical failure.
OTOH, cup & cone ones also tend to be very reliable, if properly assembled and cared for.
"Sealed" is a bit of a minsnomer. I have a couple of cup & cone BBs with a rubber seal between cup and BB spindle. No reason to think that the average cartridge BB would be particularly superior in that perspective.
'Course, I also have a few cup & cone BBs with the spindle just passing through a fairly crude hole in the cup. Obviously room for some improvement there.