I've never personally used spackling paste. I've always just used bondo, sanded the crap out of it, then used Rustoleum's Filler Primer to take out the remaining little bubbles. As per 3M's "system" they have some sort of putty I've never really messed around with, but you can try that if you want. It has it listed on the can as part of "3M's car restoration system" or some nonsense.
Yeah apoxie sculpt should still work, stuff sticks to damn near anything. I think you should be fairly liberal with your apoxie in order to make it more durable. The red is your broken piece and the gray is the apoxie sculpt. Have it lead slightly over the crack, and you should be able to sand it to a nice gradient on the top so that it matches well with the rest of the armor.
From a physics standpoint this should be stronger. I won't go into detail, but it's essentially about counteracting shear forces and bending moments equally regardless of applied loading. I'm an engineer by trade, so this is what I specialize in.
TLDR use apoxie sculpt. Use a lot of apoxie sculpt. It's awesome.
Good luck to you!!!
PS. I guess to just spit in their face you could always repair the crack, then make a mold and recast the entire thing. Totally viable since they already did the hard part (the sculpt) and making molds / casts is just following directions on the back of a bottle.
has a ton of vids if you want to do that. I learned how to cast entirely on the internet, so it's doable.
But yeah might be overkill hahahaha.