If it is fundamentally different then you have another kind of game on your hands?
I've cited some examples, haven't I? It boils down to certain systems, like stealth and combat, or puzzles and combat, having shared elements and enhancing each other despite being fundamentally different. In principle, you can even merge diplomacy with combat quite well, we've discussed this a little bit
in this thread. So no, having fundamentally different gameplay styles doesn't mean you have two games in one - it just means creating a more intricate core system.
Does it have to be fundamentally different to be worthwhile?
I've already answered that too:
Nothing wrong, but "RPG with reskinned combat" isn't the same as "RPG with no combat".
What you're suggesting is kinda like Puzzle Quest, only the other way around: Puzzle Quest took match-3 gameplay and reskinned it as RPG combat, and you're talking about reskinning RPG combat as diplomacy. It sure could make for an interesting experiment, but it would remain a kind of combat, just like Puzzle Quest remains a kind of match-3 game.