Help!

This really dependson the genetics. Some plants need to go vertical right at flip if you want a 18-24" deep canopy of good buds. Others may need training 7-10 days into flower phase to avoid outgrowing vertical space. So, there's some variance here. If you think something is a "stretcher" keep training it with the scrog. If you think somethign is a bit vertically challenged, let it go early. If any area gets congest, selectively prune when it is apparent to do so. over grown is manageable and only a few wasted watts in vege to get that excess growth, while undergrown is just wasted watts for 2+ months of entire flower phase. Best to avoid either situation, but overgrown is preferable because nearly all negatives are easily avoided.

I flip to 12/12 when my trellis is 75% full. Then i continue to tuck during half of the stretch period. Then i let the tops grow up towards the light. And thin out your canopy too so air can easily circulate. A trellis scrog can get really thick with leaves!

I typically like to train them under the first net until I can't anymore or I run out of space. The second net is not always necessary, but it will be if there is still space to fill. You're on a timer now for training. The stems of the plant will become brittle and harder to train as it continues to flower. Be careful as its pretty easy to break off whole branches. I would suggest you continue to train the branches until you can't anymore.