1. Yes, the 1.08 venturi was the size of the factory MC2100 on the AMC 304's. 45-48, because of your altitude it's difficult to say on the correct jet size. It's something you'd have to play with. I've heard of guys in the mountains running one 46 and one 47 jet, they just had to keep playing with the setup until they dialed it in.
2. In theory, yes it works. As you go higher in altitude, you have less O2 in the air so the standard carb runs richer.
To off set that, the carb has an atmospheric pressure sensitive circuit (HA circuit) that is normally closed at sea level and as you go higher, the reduction in pressure gradually allows the vavle to open and it exposes an air inlet port in the carb. This then leans out the mixture in the carb by injecting more air to compensate for the thinner air at high altitude.
3. Going to need a picture to answer this one, there are so many different MC2100/2150 variants out there, heck some of them had 2 stage power valves, I'm going to have to see it to give you a positive answer.
4. Is there a tag with a number on it on your carb? If so, I have a bunch of websites to look stuff up just for MC2100's. The only difference between a 2100 and a 2150 is the altitude compensator, so you have to make sure you're looking up a 2150.