Is there any documentation that you know of that King was a Republican? I’m not disputing it, because it wasn’t uncommon back then for people to register with a party they had almost nothing in common with for various regional or technical reasons. For all I know, Saul Alinsky may have been registered as a Republican.
The thing that bothers me is that people keep saying over and over that King was a Republican as if he was someone like Michael Steele or Clarence Thomas. And as if it’s somehow distorting or perverting King’s legacy for leftists today to invoke King. Sorry, but I see it as perfectly appropriate for leftists to invoke King, since he was a leftist both in his public stances and his less-than-honorable private conduct.
You are quite correct that King was not a conservative, and even had sympathy for Communists.
I read that he was a registered Republican in the 50s, I don’t recall the source, but I think it is an undisputed fact. Google is your friend, if you are the seeker of further data.