The LX groupset is now a touring groupset rather than MTB, and XT is also available with larger chainrings for touring

For some of your questions

Will a double chainring work with a triple derailleur? - no, you need a triple RD

Can oval chainrings (Biopace et al.) be mixed with round ones? - Yes, but you can't get biopace anymore, it's been obsolete for 15+ years

Is there an online database that lists BB sizes for old cranks? - no you will need to research this yourself, look at places like bikepedia

Is there anything particularly better about Octalink or ISIS over square taper cranks, aside from using a lighter hollow shaft? - ISIS had issues with bearing not lasting, nothing wrong with Octalink, but both systems are obsolete, you can still get BB's, but no new cranks

What's the big deal about outboard bearings? - cynically, a new systems to make money from; it is stiffer, probably cheaper to manufacture and install in a factory, very easy to maintain, your old crank won't work with it. this was been discussed before, and will be again, if buying new would go for this, if not, square taper still works well