are varsity players allowed to suit up for their jr. varsity team and play?

Yes, provided they are on both rosters. I think the rule is as follows:
you can swing as much as you want if both teams are in the same division, ie vasity and JV both play in D2. But if its separate, you can only swing up.
However, after a certain date the rosters are frozen. I think its after the semester, but I am not positive.