1. For learning modern assembly, or even embedded assembly, 16 bit DOS is not the way to go. If you really want to learn some embedded assembly, pickup an arduino/propeller/arm educational board and get started there. If your goal is to learn modern assembly, then you're in for a rocky road, but you should get started on the x64 end of things, as it gives you significantly more registers to play with.
Nintendo GBA is another excellent ARM based platform, and of course better optimized for games than the more generic arduino and such. Not much activity on the old forum.gbadev.org these days, but a few of us still check in regularly, and I'm currently working on a NES/SNES style RPG for it
ARM assembly is much more fun to write than x86, plus you can do accurate cycle counting and run on real "period" hardware... if you can still find flash carts these days.