SilverBlaze, it sounds to me like you have a natural ability to work with rabbits slowly and easily. You've done very well.

It is possible that Frost's freak-outs could freak Ollie out. You may have to win her back over as well, slowly. Hopefully, Frost will realize quickly that he isn't getting the treatment from you that he's been getting from your sister, and will stop the scrambling and screeching quickly. He may still unnerve Ollie for a while with stamping and growling. Ollie will need whatever extra attention and reassurance she will allow you to give her.

I understand about crowded bedrooms... I used to have about that much walkway in my room growing up. I'm glad you're trying to work out a way to fit Frost in in spite of the crowding, though. Sounds to me like the two are trained enough to be stacked like that. I guess the only concern would be Frost's antics upsetting the stack. Hopefully, you can figure a way to keep that from happening.

If you post pics of the cages, you might get some brainstorming from the good people here.

Cable ties stabilize anything that can hold it's own weight up. I did find you can't build 3 cages high using heavy powder coated panels even on the big ties without crashing. I'm not sure if it is good I was in the bottom cage filling hay at the time or bad. That hurt but my shoulders stopped the top 2 cages from breaking the rest of the ties and bringing the whole cage system forward. We went to pvc and wood supports for the more massive builds now but if you are just stacking things that fully support themselves you can prevent shifting with cable ties.

A board would help but you could also slide some foam on top of the board (not under where the below rabbit might be able to eat it) to cushion thumping sounds. One panicky bunny does cause confusion. A silly pest of a mini rex kept standing on my foot today every time I stopped moving and I ended up catching his hind leg under my foot when I stepped back so he screamed bloody murder despite being fine. Just temporarily stuck. Everyone went flying around in circles, except the older rabbits who perked their ears and basically went "dumb kids" and back to what they were doing, while I called them silly rabbits and brought them some hay. 2mins later it was all forgotten for the fresh hay. Throwing out handfuls of horse treats works just as well and distracts does when taking kits.