Quite possibly. However, if we think of the 10 minute constant as not actually having to stay at that constant, we can adjust it so that at the time we disable the 1 MB limit, the largest block that miners would practically want to make at that time would be 1 MB. Basically, this would protect us from having a 1 MB limit one day, to a practical 50 MB limit (or whatever is currently practical with the 10 minute constant). I mainly want people to remember that changing the block time is also something that's also able to be on the table.

Yes.

Just like you said, it would have a pro-rata reduction for increased block frequency. Sorry, I assumed that was obvious, since changing anything about the total currency created is absolutely off the table.

Yes. You wouldn't be able to trust that a majority of the network acknowledged a block until it gets past the point where all clients are required to accept it as part of the chain.

Imagine that only 10% of the network accepts blocks over 10 MB and 100% accepts blocks less than 1 MB. What if that 10% got lucky and generated two 11 MB blocks in a row? Well, the other 90% would just ignore them because they are too large. So, those blocks get orphaned because the rest of the network found three small blocks. If you just accepted the 11 MB blocks as a confirmation and sent goods because of it, you could be screwed if there was a double-spend.