Speaking of Perrin being a mope, it was interesting to read a criticism in which I learned (because I haven't read the books) that Perrin doesn't have a guilt crisis in the books until he kills later on, during their journey. In the show, he accidentally kills his wife before we even get to know him, so he starts out as a mope, robbing him of important character development and viewer interest. It seems like it might've been a poor judgment call by the show writers.
He might've not returned because his character left the main group and didn't have any scenes in the final two episodes. Seemingly so much of the book has been changed that the writers might've planned it that way and it was just a coincidence that COVID after his scenes were finished. Your theory (that the plot was changed because he didn't come back) is certainly possible, too, but we don't know.