I've only looked into categorized answer display page so far; it uses editinvote whereas categorized question display page does not, and never has.

It looks to me like editinvote depends, and has always depended, solely on canUpdateNode for its authorization check. And canUpdateNode does not -- and I guess never has -- accounted for $NODE->{original_author}. So I'm pretty sure I was right, at least as regards CatA nodes.

And given that, I'm also comfortable in my confidence that it was also the case for CatQ nodes.

Further investigation (which was surprisingly tough; very little seems to have been said on the specific point of the editability of CatQ/A by original author) -- I found:

• This old Editor Request, which was asking for an edit to a CatA -- which wouldn't have been necessary if the author could have edited it himself.
• This old PMD subthread, in which monks discuss the idea of opening up CatQ/A nodes to editing by author. vroom (and others) explain why it was not done that way.