Reward the sit. Or rather, mark, then reward the sit.
Using a reward marker properly will make it clear to her what behavior you want - if she starts a jump-sit cycle just space your mark for a slightly longer period of sitting. One other thing that works
really well for encouraging excited dogs to stay down is either tossing or placing the treats on the ground.
Attention, even negative attention, with a bored and hyper puppy is going to make her more aroused. You are on the right track. A vocal reward marker is fine if you make a consistent sound. I like 'Yes' because the consonant 's' sound tends to get naturally emphasized - an 's' hiss is very specific and more of a sound than a word.
She sounds like an excited puppy trying to figure out exactly what you want of her (and becoming a little frustrated, but that's very understandable given the circumstances). Is there a fenced area at the shelter where she might be able to do a few zooms off leash? That + a bit of training will be way better than a leashed walk.
In the mean time, my suggestion would be a slightly longer leash that you could step on, not to hold her down (no pressure when she's at normal dog height, we don't want her dependent on it) but to stop her from getting any height with any jumps and grabbing you. Ideally, a second person to hold the leash until she kicks the habit, or it at least becomes less extreme, would be handy, but work with what you've got.
I would also practice removing her from her kennel (at the start of the walk) as conditional for calm behavior. I'm not sure if this is part of the problem in her case, but it certainly was with most of the shelter dogs I have worked with.
If she's jumping, she doesn't come out until she calms down - it might take a few sessions of practice for her to settle in any good time. In this situation, you and the open door are the main rewards, so be ready to remove yourself and close the door if she starts to get rowdy. For clarity, you could add a no-reward marker here like 'oops' or 'that's not it' to make it extra clear.
Once you know she has the sit down, you can ask her to sit while she is bouncing (say it once and wait for her to bounce herself out), reward with food to keep her down (toss it to her) then reward her further with an opening door and you moving closer. If she can stay calm(ish), you put on the leash and lets go! If not, go back a step until she gets it.
You are doing an awesome thing
