I'd like to have my young grandson experience the Astronaut Training but I'd be excluded from some activities due to a medical condition. Can my grandson still do all the activities even if I will only do some of them?

Depends on how old and big he is

I didn't see any limitations noted. I think as long as you're there and available, he'll be fine to go. I'm thinking this is more of an "emergency contact"/"sign this release" type of requirement. But I'll add that you can call and ask. They have a whole section that does customer service.

Children less than 18 must be accompanied by a participating adult. What do they consider "participating"?

How old is your grandson? I did not see very many limitations when we went and we had a young explorer of our own when we went