If you want to read the Joker story in Batman 286, it is scheduled to be reprinted the Joker Bronze Age Omnibus.
As for Mr. Freeze, by this time, I think he had made many more appearances on TV than he had in the comics. As you mentioned, he was on the 66 series three times and multiple times on the Filmation series. I may be forgetting one, but I think he only had two comic book appearances by 1977, not including his cameos in "Where Were You the Night That Batman was Killed?" I think Batman 308, written by the great Len Wein, was Freeze's only starring role in the 1970s and it wasn't released until after the Filmation series. This story also had a humanized Freeze and I wondered if it influenced Paul Dini's later take on the character.
Really, I always figured the only reason Freeze made it to the TV series was that his first appearance, when he was Mr. Zero, was reprinted in Batman 176, which Mike's Amazing World says went on sale on October 7, 1965. I always figured that was one of the issues that William Dozier read, since it also features The Joker's Utility Belt story that was adapted into an episode. That 80 page giant reprint issue also had a Catwoman story, and she hadn't appeared in years, and a Penguin story.
As for Clayface, he was, indeed, an ideal opponent for the show. The cartoon was my first exposure to him.