Cells discharged to that low of a voltage will have permanent damage, the internal resistance of the cells will be much lower.
Please do not try to recharge your pack. Even if you are successful (unlikely) the risks involved in the process are high.
I have personally recharged cells (not packs) from sub 1v, some will maintain a charged voltage of 4.1v. However when tested the cells max discharge rate and capacity were greatly impacted (~20% of original).
Due to each of the cells having different resistances charging them as a pack is a very bad idea. Let's say you have a pack with 2 cells in series and you provide them with 8v. If one of the cells is damaged all of that voltage will be going into the good cell.
I know it's not what you want to hear but I would suggest reaching out to Zero about a replacement battery pack. Not the cheap option but a lot less expensive than your home. There is a DIY powerwall group on Facebook, if you want to see the results of thermal runaway.
Point taken, I do understand the basics of cell balancing and on a much smaller scale have revived battery packs by addressing each cell (18650s) individually and removing the duds.
I will attempt to contact Zero regarding options. My bargain bike may turn into an MSRP bike at least I would suspect, so at that point I will have to decide if I should cut my losses and sell it as-is or forge ahead.
Thinking ahead, especially if there is no refurbish or core charge option, what cells comprise one of these? I saw a youtube from several years back repacking one from a Tesla array but it was a different bike and tech changes rapidly. As I say, just going through the options in my head.
Thanks again for the input