If the relief valve went bad and is stuck open then oil isn't reaching top of engine.
If oil isn't reaching top then everything is drying out, trying to find why that cam chain got stuff like that.
You could be right that coolant was low and things got hot. Also if cam timing was off like you said that could cause excessive heat to linger longer in the top end than is healthy.
If it was me, I would be pulling oil pump and relief valve to inspect when I was this far in, if you're doing piston and chain already I would chase down any potential cause to no oil travel before I put it together and risked all my new parts.