Did you over torque the rocker arm? If it was too tight it may have bound up and when the cam went to push it, it snapped.
I don’t think temp would have had much to do with it, if it went on a fourth heat cycle (cold to hot then back), then better it went in your driveway than on the road.
I guess try getting a telescopic magnet and try to fish out those pieces. It would suck to have to tear open your engine just for those pieces, but it would give you an opportunity to clean everything throughly.
Best of luck, sounds like a tough situation.