Is it even possible to re-vegg a fast flower?

Probably not due to the infusion of autoflower genes into the fast flower mix. Personally, I would stop hoping for a miracle and just start new seeds.

The question is, is it worth it. I'd say no...

it's a photoperiod? Yes, you can re-vege it. The 'auto' gene isn't active in those. they just think they are making a faster plant even though it isn't any faster. re-vege takes forever and a day. It won't drink much but you also can't let it dry out. the leaves aren't dead, so... keep going. you don't need 20/4. you just have to avoid giving long uninterrupted darkness as always - nothing new here. Do what you did in vege phase. don't over-think it. Less than maximum might be best for now until growth returns. if you were giving the same intensity to the plant over 20h that you gave over 12h, that would be a contributing factor - i.e. not helping but hurting.

My cousin planted in the garden, it was outside, not an indoor. She harvested for the first time, took almost all the buds from the plant, but didn’t take much from the branches. And the plant stayed outside in the winter, and it’s incredible that it survived, it started to grow more in spring and summer, bloomed in autumn and winter, when it was finally harvested for the second time; however, it didn’t manage to survive a third season...

If you want more experiments and observations, sit down a new seed. There is nothing interesting with this.

Yes, if it,s reg or fem