Overflow bottle melting

1 Are you using waterless coolant e.g. Evans? It can get much hotter than water/glycol mix.

2 Is the bottle's vent hole clear - could pressure have contributed to the split?

Why does there appear to be a line going into the top of the bottle?

You said you're aiming for oil temp of 100 degC to boil off condensation. I've always heard that 190 degF is sufficient to do this. Depending on several factors, I may have to push my engine much harder to get to 212 degF. Could you be pushing harder than needed?

I use water glycol 50/50. The tank is vented at the top so no pressure problem.

That is a breather vent