1) Technically not very legal, but if you do it for private use (i.e. never share with anyone, because else you pretty much violate many laws) then well no one would care (as long as they don't find out you did that to begin with).
To do it, you first need to figure out what goes into the engine, aka the compression type. There are several methods so you gotta figure out which one is it. I suspect I know what it is but there's more than just compression.
The biggest thing you have to know is that if you want to decompile *any* engine is that you aren't going to get 100% of the data. You'll be happy to get even half, maybe more.
And it's not just one simple step, there's quite a process and that could take a few weeks/months on its own. You aren't going to find a decompiler either, it's something you have to build yourself manually.
Additionally, MB's engine was heavily modified for MB and pretty much differs from any other Torque Engine. Go with the fact you aren't going to get 100% data (as said) and you get pretty stuck. Likewise, the engine misses things like terrain, water, AI and other things (mind you, shaders didn't come until later versions/engines).
You're better off getting TGEA and working on that engine and doing your own physics for it... will be much faster and has all the stuff you need (terrain, AI, water, shaders, multiplayer, etc.).
2) I don't know here, but then again, MB9 uses code from MBU which is illegal so I can't help you there.
"matan, now i get what you meant a few years back when you said that "the level in mbg is beyond me" after the last rampage i noticed things were insane, and now i truly feel that too" - Dushine, 2015.