Ahh charisma skill monkeys, always infuriating to deal with as a DM

I think it's important to determine what characters will compromise on and what they won't. If people get worried about their planned out stories getting ruined because someone swapped sides? Oh noooo *waves hands sarcastically*
This one here has a perfectly reasonable result. Companions that were treated well in the past (including literally moments earlier) going with someone against a big organization that might kill them is good.
Now, if you've treated someone poorly for a long time, a dc 100 or 200 skill check shouldn't do much of anything.
Related: my most recent campaign to finish I had someone who wanted to be friends with everyone. He managed to turn an entire army around after the party wiped out their king and archmage. It was half intimidation half persuasion, because they were marching through an absolutely terrible region the PCs could control, and he had the perfectly good excuse that
1. They didn't want to be there
2. They needed to go back to their capitol because someone was doing a coup while they were gone
3. He had a dragon.
Diplomacy and intimidation are all about circumstance bonuses. If anyone shows you flat DCs in a book, ignore them because they'll never make sense.

Anyway, I personally prefer people who actually put in the effort to get the skills, and then actually act in a way to make those skills make sense. Then again, I'm not the sort of person who plans out an entire campaign because I know PCs will either wreck it or I have to tell them they can't do things they should be able to do... and that's with fairly accomodating players overall.

That's just it though. The difficulty with charisma skill monkeys isn't so much "does this work or not?" it's "How do I portray that their character is extremely convincing, but that the person they're trying to convince can't be swayed?" The balance is very hard to maintain between circumstance and skill bonus