'putting heavy load on a drivetrain with the rear derailleur not in line cause a chain breakage'. Yes...as is obviously the case Had it happen to me with a 1 x 10, trying to change up the block on a short sharp 7% climb (indexed). It's not that the indexing fail to do it's job, it was me - the rider - who was forcing it to do it under load. As you were using friction shifts your magin for error is narrower considering the nasty distraction of a car door. It's not uncommon. The outer plate on one of the chain links tore away from the pin...It's only happened the once. Before that I'd never broken a chain in over thirty years of pedal pushing.

As for the rear mech being damaged, well a visual inspection of it's postion on the hanger and the position of the hanger itself is in order. The only true test, if it looks ok to the naked eye, is to slap another chain through it, pop it in a work stand and cycle the gears...If a mech is seriously damaged it's usually pretty obvious to tell, normally because it out of shape (jockey wheel cage twisted, snapped body pins and the like).