Probably been answered elsewhere, but eARC allows for more audio channels and higher bandwidth for immersive bitstreams like DTS:X and Atmos. It has other features which may or may not get implemented as well.
Re:
I've not tried IMAX Enhanced on a flat panel, as I fear that it may cause irreparable damage, and OLED panels don't come cheap.
I don’t understand this comment, how would this cause damage?