Dungeons and Diplomacy
I preface this by saying, whatever you rule kinda goes. As such, don't ever force a decision on a player. If they have stats, your players may well want to kill them regardless of how silver their tongue is.
Mechanically, 3.5e is well suited to the idea that not all monsters are villains and not all villains are "evil". So some things to consider aren't necessarily about how your players might handle a +32 diplomacy but how other NPCs might handle it. Also consider that monsters are human too. They have needs and means to achieve them. So looking at it through these lenses may provide a better perspective on how you might roleplay a high diplo character.
NPC to NPC interactions
Let's say your players are going after Ivan Von Badguy, one they know to be a dangerous vampire lord. But he hasn't really done anything that bad on a world or even kingdom level (ok he murdered your buddy's whole royal line but I mean that doesn't count). Well when you get to the border kingdom you hear about all the good trade missions that have been done between Lord Badguy and them, it might change some perspectives. Or posthumously, when multiple different kingdoms cry out for the heads of the vicious assassins that murdered Lord Badguy in his sleep, those consequences might really settle in.
Groups and factions talk to each other. So an easy way to look at diplomacy is to look at its outsized effect. Trade, alliances, etc. all depend on diplomacy and are easy ways to narratively announce the power of a "monster". And best of all, it doesn't force your characters into anything. There's no "well he got a 30 on diplomacy against your 15 so you stop fighting him", just a simple nudge that the NPC in question might have stronger and stronger back up or diplomatic pull.
A Means to an End
There's a skillset for every occasion in DnD. You have terrible brutes, crafty experts, and charming faces. Of course, a party can specialize and spread out its skills as needed. But every now and again, you need an S-tier jack of all trades. One who solves all problems. And so too do NPCs. The most tyrannical dictator dragon lord and the most devout angel of the gods both might just need to take a personal hand in any situation.
Consider again the Vampire Lord. Sure they have thralls which could be used for low level diplomacy. Securing meet and greets, sending invitations, even operating semi-independently as spies or courtiers. But when it comes to high stakes, that's when they'd show up. Securing dubious alliances, preventing (or provoking) war, etc. is when Ivan would be needed.
Making Mechanics Happen
It's all easy enough to do things in the background. Putting diplomacy in the background as you go from low diplo to high diplo characters is an easy way out. But when it comes to actively making diplomacy "feel" like a skill it's a lot trickier. How I do it is to have an outline of the NPC goals. Whenever that NPC is talking, stick to the outline. That NPC should project more confidence and assuredness as they "gain" diplo levels. Actively start keeping yourself from filler words (uh, hmm, well, like, etc.) when utilizing them. At a certain point, like with a Formian Queen, I would start fully projecting my voice. They are powerful beyond mortal measure, more knowledgeable and wise than many great sages, they should have no doubt about anything they say. They are imperious and even if they are good they should still be seen as intimidating.
So in summary, a way to roleplay out higher and higher diplomacy is to look at its outside effects from PCs where they are known or rumored about before characters meet them. Additionally, you should place them in higher stake situations where a "normal" diplomat might not do. Finally, use your full chest when utilizing top end diplomat monsters. They have strong personalities as they would need and should always be offering full throated statements.
