I can't comment on whether its the plastic or not to blame. I can state for certainty that removing the two rear staples releases the door panel from compression/tension forces and relaxes to the point that you can just position the cracked portion with one hand to where the crack is barely visible. I imagine that some careful light application of vinyl/plastic paint in the correct shade would help conceal the crack after the repair has been performed.
I recommend removing all the staples. I am not sure what I will use yet but I am considering using some small zip ties to hold the inner window sweep on and epoxy a patch on the back of the crack.
See photos for two staples removed before and after Staples removed AND crack NOT aligned Staples removed/crack aligned. Some light application of plastic/vinyl paint should make this mostly invisible.