I like these techniques and good write-up. There are a TON of offensive moves against the turtle.
Another transition to the back that I use a lot goes like this: Capture around the hips with both arms as to hug your partner then placing knee to knee hip to hip facing the same direction as your partner, with outer leg flagged out pulling partner onto your base.
This is the control position and is advantageous if your partner feels threatened and tries to roll, spin, sit-out, or what have you. As you have their hips controlled and are in a more mobile position.
If they continue to turtle: You may use the leverage of your combined arms and shoulder/torso rotation to drive their elbow apart from knee and in doing so place your once flagged out leg in between gaining the hook. At this point you are now above them and may proceed to gain your second hook. Be sure to stay tight as if they decide to roll you will stay with them. Once both hooks are established you will work for the over/under seat belt grip.
There are three advantages to this method: 1. You have a very dominant control position to start out with 2. You can dictate more effectively where you wind up on their back (high/low) 3. It is a good method against someone who is very tight in their turtle.
Maybe this is a pretty standard transition but I haven't really seen it very often.