Diddy Kong's last major appearance in a "main" series DK game was DK64 all the way back in 1999, nearly a decade before he made his playable debut in Brawl. His last appearance in general was just the year before, in Barrel Blast.
King K. Rool's last major appearance was also in DK64, though King K. Rool (and Dixie, oddly enough) both appeared as trophies in Melee, with Diddy only getting a passing mention in Dixie's trophy. K. Rool also appeared in Barrel Blast, just a year before Brawl. His last appearance in general was in Mario Super Sluggers, the same year as Brawl's release.
Relevancy matters to an extent, but fan requests certainly have a large effect. As you mentioned in your rather large post, "retro" series apparently don't count, I'll give you some leeway there, but take into account Mega Man. Mega Man is quite obviously a third party character, one that has been "abandoned" by his own company, similar to K. Rool's situation, yet Mega Man is in Smash nigh solely on fan requests. Sure Mega Man falls under "new series," but that's a new series that requires a lot of legal work and royalty payments, it wasn't as easy as adding someone like R.O.B. or Mr. Game & Watch.
I'll use your previously linked post again, in which you mention Lucario. While a franchise like Pokemon is a completely different beast from Donkey Kong, there is still a valid comparison to be made. While Lucario was indeed added due to "recency," we also got another character in that same game, rather, three characters. The Pokemon Trainer, and his three Pokemon "all the way back from" gen one. Sure you could argue that he represents the generic player character that you still use in newer games, which is true, but he is specifically modeled after the first one, and uses three old Pokemon.
I'm a big fan of our King as well, but I hardly think he's a "shoe-in," I'm honestly dreading the moment he shows up as just a boss, but I do have hope, and I'll continue to have it until he's either confirmed to be playable or not.