when attacking the turtle from behind, how do you stay behind the opponent?

When I'm on their back, the opponent may manage to grab my arm and try to pull me over to the side. Is there a good counter-attack against this defense?

You will get the best answer from your sensei. It is hard to know exactly what you mean from just a brief description, but, here are a few ideas that might be relevant:

1. get off their back so they don't control your mass -- sprawl most of your body off to one side (while still directing your weight down into them). it will be harder for them to pull you over to the other side.

2. if you then decide to let your arm drift over to their far side for some reason, try not to let them grab it above your elbow. it will be harder for them to pull you over to the other side.

3. ... but if they still manage to grab you lower down the arm, eg. by the wrist, then you can LET them try to pull you over to the side, and this can be your counteract. They can end up pinned, with your arm trapped underneath them, which really locks them down.

you will have to practice it of course, good luck