Sometimes when you think you understand something is when you haven't thought about it enough (referring to myself). I had missed a couple of details here at least.
Update: yeah, I was definitely confused. The rest of this is just silly, well except for the bit about the zener voltage sensing:
The zener situation is probably quite a bit different than the LDO situation. In either case though when ground is disconnected the mcu should continue to run powered by the cap(edit: I really was thinking about C1 here confusing it with OTC). The current draw on it is I guess 10x higher than the divider current so it runs fast until it reaches shutdown voltage in zener case or the ldo reaches its dropout limit in the ldo case. But this is fast compared to the divider RC anyway so doesn't contribute a significant lag. Obviously it impacts the t=0 cap voltage.
In the zener case I hadn't appreciated that the cap ) stays only at zener voltage (plus or minus the input diode vf) according to this diagram anyway:
In the LDO case it stays higher and for voltage monitoring while on that matters, but when turned off it still should drain down to a pretty similar level, but the voltage of the LDO should have a big impact on OTC timing I think, because it determines what voltage the cap is left at when it drops out. It seems this has to be higher for LDO than for zener so maybe there's some confusion comparing OTC when there's two different situations.
I assume the LDO and mcu shutdown with high resistance between vin/vcc and ground, but maybe that's different for the two cases also.
Now what I really don't get is the turn on. In either case if the mcu/ldo has enough voltage to turn back on, the cap must have already charged back up no? And always to the same level. So what's left to measure? It seems to me almost like a resistor is missing between the R5/R1 juncion and the cap (or between the cap and ground). Shouldn't the cap charging lag behind Vcc if the cap level is to be detectable? Maybe that would help with some of these "stability" issues? Obviously it works, and I'm still missing something. Still maybe it works for reasons that aren't well controlled? Or more likely it's just me.