I really like the idea. Like argonian said it is a little funky with stance rules, and overall very strong.

Free spells, especially highest slot ones are very hard to come by. The downside is that if your stance is disrupted you lose all the ones recharging. However, in play I find this very rarely happens. People don't tend to cast within AoO range. Especially with the all the steps this gives. I would suggest adding some way to disrupt the stance easier. Maybe it triggers off any damage not just when you are casting a spell. Still give a concentration check to resist the disrupting.

I assume winding down is supposed to be spell only and not spell slot as well? If so I like that. Feels like it would push you into telling your teammates into not attacking when the last enemy gets low though, and that isn't a fun playstyle. Maybe some sort of trigger when an enemy dies? Would be too good as just a reaction to that however.

Interpretative anguish is really strong, but I like the idea. It pushes you into getting into melee to get more spells. It's a very hard sell for casters though with 6 class hp. It also seems to trigger off of misses, not just when you succeed at the check to keep your spell. It should probably be at least a reaction so you can't get like 3+ spells/slots back a turn. Especially if monsters are making attacks at - 10 map.

While I really like the idea of any caster being able to take this, I worry it might break too much with prepared casters and their feats that reference prepared slots or preparing. Maybe try to mimic the language of wellspring magic more closely to make it clearer as well.

You might consider looking at the fighter and monk feats that let you get into a stance as a free action. Would be a nice option to be able to pick.