Matching BB to new but discontinued M442 crankset
I'm upgrading an old MTB frame (Giant Alu Lite 6061) with a new crankset - a Shimano M442 square taper. The Shimano tech docs suggest the Shimano UN22 BB. Some people claim the UN55 works as well. I'm confused about brand, model, and spindle length. The old crank was a square taper 3-speed. Forgive my ignorance - here come the questions.
1. Do I need the UN22 or can I purchase any square taper BB (Shimano or third party)? I see the UN55 has a lot more positive reviews, but is it really an improvement over the UN22 or are there simply more reviews due to it being newer and more present in this age of Amazon reviews. I've also found some generic and third party square taper brackets that seem to match the UN22's measurements. Is there an advantage or disadvantage to straying from Shimano's recommendations? The chainset im purchasing is discontinued, so its only natural to assume those docs will recommend whatever was in production at the time of writing. BMW and Apple recommend their own display and audio cables but I've fared pretty well straying from those recommendations (except for my aux in adapter - BMW wins there).
2. Spindle length. Is it safe to match my old bottom bracket's spindle length to my new BB spindle length since I'm replacing a 3 speed MTB crank with another 3-speed crankset?
3. How crucial is spindle length? Can't most things be compensate for with spacers and front derailleur adjustments?