Thank you I was wondering myself. So if you don't want to ride/can't ride, then you basically just exit into the gift shop and only miss the ride and the ride loading. Is that correct?
Correct (but I do believe you miss the Sorting Hat. I don't remember seeing the Sorting Hat)
But, there is also a good chance that right before the exit to the gift shop (right next to test seats), there will be a TM that will nicely approach you about trying the test seats. I tested the seat even though I knew it wouldn't click three times (tried the outside seat), but you could probably just tell the TM you wanted to experience Hogwarts and they'll direct you towards the exit.
I have to say, I was very impressed with how the TMs I saw dealt with the situation. I have a feeling that they realize how restrictive the ride vehicle is and were quite apologetic to me.
There doesn't seem to be a weight requirement...yet. It appears (and was verified to me by someone in Guest Services a few days ago) to be yet to be determined. That is if they can even come up with an actual weight. Body shape seems to be more important.