TLDR Version:
1) This IS NOT an inescapable reset, delayed reversals beat an early timed meaty Flash Freeze, while early reversals beat a delayed timed meat Flash Freeze. This needs to be fleshed out more. I haven't had the time these past few days. It's my Sunday project.
2) It's character specific - I'm not sure of the final number... 7 comes to mind. Many (most?) characters aren't even worried about it. I'll compose a finalized list of victims soon.
3) It sweep version works on the right hand side of every stage, except Watchtower Bridge (where it works on the left)..... The Flash Freeze -> Flash Freeze reset works on 2 stages: Batcave and Atlantis.
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For this write-up we'll ignore Watchtower Bridge and Themyscira Port - both of these stages are odd and basically deserve their own breakdown.
Point 1
Batcave Lab and Atlantis have unique interactions with their right corners that allow "the loopable reset." None of this cool stuff works on the left corner of any stage except Watchtower Bridge - which I'll explain in another post.
The "loopable" set-up: flash freeze -> meaty flash freeze: did they reversal? no! do it again!.. did they reversal? yes! punish with flash freeze... rinse and repeat. This is character specific to VERY FEW characters whose only viable reversals have enough whiff time to guarantee a raw flash freeze. Batman and Zod are most notable.
Many characters whose reversals do not beat this set-up - Frost, Joker, Lantern, Superman, Lobo - are still susceptible to a punish of some sort that combos into the Flash Freeze. Some can reversal super, some can't. All of them can Clash, while Batman and Zod cannot because of the nature of the reset.
HOW TO ESCAPE
However, as I stated in the original post that started this thread, delayed reversals are probably immune to the reverse wake-up thing and should be tested. Sure enough, they are. If you delay your reversal, with say Batman, your slide kick will come out in the proper directions and beat the meaty flash freeze. So, hope is not lost.
Point 2
Most other stages you can still perform this reset off of a sweep. For some reason the corner interactions on most stages make it so the Flash Freeze hard knockdown isn't exploitable by this tech, like it is on Batcave Lab and Atlantis.
What this means is, if you have the meter to spend, you can end your corner combo in sweep(d3) (there are many options for this). After which you perform a meaty Flash Freeze that has all the same effects as the above set-up. If the meaty Flash Freeze connects, you need to spend a meter to loop the set-up.
Once again, you can beat this with a delayed reversal. So, no, it's not a perfect trap. Thankfully/Unfortunately, depending on your perspective.
!!!!!POINT 3!!!!!!
Anyone who took the time to read through this thread knew everything stated above. What hasn't been discussed yet is that there seems to be a 6-8 frame window for timing this set-up. You don't have to be perfect to time the unduckable, unjumpable, unbackdashable Flash Freeze, there are a few different timing's that work.
Meaning? The reversal window varies depending on the Flash Freeze timing. If I perform my Flash Freeze as early as possible, you need to perform you reversal as late as possible. If I perform my Flash Freeze as late as possible, you need to perform you reversal as early as possible (and it seems like there's atleast one other timing where the opponent can only beat the Flash Freeze if they time their reversal somewhere in between early and late - this needs more testing to confirm).
And if I don't Flash Freeze at all? If I just block, or f3/b1u3 or jump or whatever?
It's a mix-up, a really really good one.