Well Ben, it's got to be remembered that many muzzleloaders had iron barrels. Some were steel, and the ones with steel barrels might be very strong or somewhat weak compared to modern gun steel. So if all things were equal, it's true that a muzzleloader can be loaded heavier then a breach loader, but things are not always equal.
I suppose that a newly made breach loading double could be made of steel so strong as to equal a good modern muzzleloading barrel, but I think the point is moot. I am assuming you are speaking of doubles and singles here, because if you were talking about a bolt action your question would not apply. Now there are going to be exceptions. The Ruger #1 for example is so strong that it's almost off the scale in comparison to most other rifles.

#2.
I have never heard of one going off while it was being loaded. I can't see how it would be possible if done in correct order. After all, the cap is placed on last.

And to answer your third question. If you are talking about a 6 bore rifle firing balls I would guess the standard load will be about 300- 350 grains. If you are talking about a bullet gun I would have to guess it to be about 265-300 grains.
However, there were very wide variation in loads back in those old days too, so these are just averages.

I hope this is all helpful