Sorry I explained it poorly and emphasized the wrong thing.
The way it would work is not active destruction of data just a different view of data that doesn’t include any metadata that is encrypted in second profile.
Data would get overwritten only if you actually start using the fallback profile and populating the "free" space because to that profile all the data blocks are simply unreserved and look like random data.
The profiles basically overlap on the device. If you would try to use them concurrently that would be catastrophic but that is intended because you know not to use the fallback profile, but that information is only in your head and doesn’t get left on the device to be discovered by forensic analysis.
Your main profile knows to avoid overwriting the fallback profile’s data but not the other way around.
But also the point is you can actually log in to the duress profile and use it normally and it wouldn’t look like destruction of evidence which is what current GrapheneOS’s duress pin does.
The main point is logging in to the fake profile does not do anything different from logging in to the main profile. If you image the whole thing and somehow completely bypass secure enclave (but let's assume you can't actually bruteforce the PIN because it's not feasible) then you enter the distress PIN in controlled environment and you look at what writes/reads it does and to where, even then you would not be able to tell you are in the fake profile. Nothing gets deleted eagerly, just the act of logging in is destructive to overlapping profiles. This is the only different thing in the main profile. It know which data belongs to fallback profile and will not allocate anything in those blocks. However it's possible to set up the device without fallback profile so you don't know if you are in the fallback profile or just on device without one set up.
Hopefully I explained it clearly. I haven't seen this idea anywhere else so I would be curious if someone smarter actually tried something like that already.
What you say makes sense, just like the true/veracrypt volume theory. I can't find the head post to my "that's why you image post" but what concerns me is differing profiles may have different network fingerprints. You may need to keep signal and bitlocker on both, EVERYTIME my desktop boots a cloud provider is contacted -- it's not very sanitary?
It"s a hard problem to properly set up even on the user end let alone the developer/engineer side but thank you.