[popnbrown]

1. Legally I don't think there's anything preventing you from changing rosters at any point. Morally..is another question.
2. Depends on the teams and the students. Will the kids on 9256 actually contribute? Can they contribute? Do all the 8811 kids get along really well? Would the 8811 kids that get removed be completely demotivated? Often going to the next level of competition can trigger motivation.
3. Any amount. In fact, I highly encourage them to help their fellow team. Of course, the limit is to not do it themselves but to rather work with the 8811 students. Perhaps you can have a grand double team brainstorming session where the 9256 kids can present some ideas and suggestions to 8811.
4. Same as #3, as long as they aren't doing it themselves and are kind of in a more "mentor"-like role that would be the best. I think this is a great opportunity to train some of the 9256 students to develop some mentor skills (which can help to mentor other rookie teams).

It depends on what you envision the program as. Do you see the two teams as separate entities, where they work together occasionally but are primarily individualistic? OR do you see the two teams as one team, just that you get to build 2 robots, and students can interchange constantly between the two teams?

I've never coached two teams so I can't say what I would choose, but some trial and error would be my way of determining things.

I will say, that for judging purposes it's more favorable to see two separate entities. For example, having one engineering notebook for two teams is probably not a good thing.