Why add the TL431? Doesn't it just inject a delay due to its response time?

Why is your triac trigger on the high side of the fixed voltage? Doesn't that defeat the purpose of the trigger circuit, namely to sense the increase in voltage at that point?

Normally one would put the fixed voltage above the thyristor gate so that, when the cathode voltage goes up, the anode voltage does to, causing the thyristor to trigger.

There exists a whole range of sensitive-gate SCR's intended for exactly this purpose, and it would seem to me that you could omit the regulator and one resistor, simply relying on either R1 and R2, or, better, a zener in the place of R1 and attaching the gate to the junction between the zener and R2. Then, when the voltage on the battery exceeds your threshold, the SCR will trigger.

Triacs are relatively slow, while SCR's are not. The benefit of using a thyristor over using a mosfet or other semiconductor is that the thyristor will stay "on" so long as current flows. Consequently, you want your thyristor to be as close to the fuse, i.e. upstream of any potentially affected components as possible. I like 'em best when they're upstream of any regulators or what-not. That way, I'm not using a more costly component to protect the fuse.

I'm not convinced this arrangement will do what you want, and I'd guess the cap (C) will just slow things down. Do you want that?

RE