So, I've been thinking about the act of persuasion in ArM. You know, getting the milk maid to sleep with you, convincing the knight to lend you his horse, persuading your master that you didn't peek in his spell book without permission, or making an argument that these woods really aren't that dangerous, and the grogs should follow you into them. But ArM 5 doesn't have a Persuasion ability. The best I can figure is some nexus of Bargain, Charm, and Leadership. None of them fit perfectly, and they all come up wanting in some way. But they're the best I have.
What have other people done for cases of persuasion?
I don't think there should be a single "Persuasion" ability.
Having many abilities to influence others makes for a more nuanced and fun game.
To get the milk maid sleep with you, you can use Charm (to seduce her), Bargain (to offer her a deal she can't refuse ) and possibly many other abilities, depending on the situation -- Guile, Leadership, Intrigue, Carouse etc. And Folk ken always helps! Each approach is different, and colours your character differently. The same goes for convincing the knight to lend you his horse, or the grogs to follow you.
Convincing your master that you did not do something is most likely Guile, though. Note that this is a subtly different notion of "persuasion" -- convincing someone of the truth of some statement, rather than convincing someone to do something.
ArM5 has - AFAICS quite intentionally - no universally applicable persuasion ability. I wouldn't see how such an ability could fit with the middle ages either.
Pure medieval wordcrafting is taught as Artes Liberales (Rhetoric), and as such isn't likely useful for the cases listed in the OP. Practical rhetoric in an ecclesiastical context benefits from academic rhetoric, and is covered by Profession: Preaching (A&A p.13f, TC p.130). Around 1200 it was still a novelty.
Most ArM5 general abilities to influence people rather improve understanding and control of specific types of situations. They rely on Prs more often than on Com, but typically have also applications based on Int and even Per.
TME the most generally usable is Charm. A guy relying on just a quick tongue to get by might have a high Guile (Fast Talk) - but the milk maid he fast talked into bed is likely to hate him for the rest of her life.
I don't think the proliferation of Persuasion-type abilities is a good thing. I'd be very comfortable with a single Persuasion skill, with specialties such as Fast Talk, Charm, etc. Right now the skills are much too granular, compared to wider ones like Ars Liberales.
It really comes down to personal preference, and I must say I personally far prefer it this way. A single ability that's just a yes/no on persuading people to do things isn't nearly as engaging as using one or multiple of a variety of skills to glean information, improve people's perceptions of you, and take social control in varied circumstances, and wrests a lot of roleplaying opportunity out of players' hands when they try to apply unusual skills to their circumstances and play characters with differing social strengths and weaknesses (much like people in both real life and fiction have varying social strengths and weaknesses). Plus, it's kind of nice just having a game with the combat-magic-social paradigm where the social aspect isn't the neglected one.
I like that the social mechanics have so much flexibility. Sure it's a bother if you try to design a character who is universaklly good at "persuasion" because he or she'd need a boatload of expensive abilities, maing it difficult to get the 'booster virtues' for them all. But IMHO it gives more variety that you need to decide how a character is persuasive.
When we run social encounters - which happens a lot in our saga - players decide which approach they use, which ability they use and this affects the scene.