PEX pipe burst: what else needs to be fixed to prevent it happening again?
PEX pipe does not corrode or otherwise interact with copper or other materials the way dissimilar metals do.  It's essentially an inert substance under normal circumstances.

The bulges and darkened areas, however, do appear to be signs of a problem and I agree with your assessment that these were caused by heat, probably from the torch used to sweat the nearby copper pipe joints.  The person installing the copper should have taken precautions to prevent heat from getting to the PEX pipes but clearly did not do so.

The damage has been done and the bulges show that failure is likely since it's the water pressure and NOT air in the system that is causing the weakened area to expand.  Once that process starts, it will keep "blowing up like a balloon" until it fails.

The damaged areas should be cut out and new sections should be put in.  Based on the obviously careless installation of the copper, an end-to-end inspection is called for.