Welcome to the forum.
>1. Can you stretch too much or too hard?<
Yes, too hard is a problem.
>2. What kind of pressure are you supposed to put on your unit while jelqing?<
A fair amount. If your penis is extended for some period after a jelqing session, this is a good indicator. If your penis shrinks back, this may be an indicator of too great pressure. You need to allow some blood to flow back past your grip but the point is to increase pressure ahead of the grip in a wave.
>3. Has any one else gone a couple months with zero gains then started gaining?<
Yes but you you should now look in detail at your routine, and whether you are commiting to the routine.