doctorbadwolf
Heretic of The Seventh Circle
Since you can see it in Stealthy, I think you already get what I've been saying. I just see it in the others, too, while you don't.
The Diplomat feat just reads as advice on how to put mechanics behind any a standard attempt to persuade someone into friendly feelings.
Same with Menacing. It's good advice for a DM trying to figure out how to decide what being intimdated means, but even then it won't likely be a good fit always. But there is something good about this feat. I'd prefer if, for example, it had said "in combat, you can give up an attack to try to intimdate a humanoid." Period. I'm good with that.
I don't see it in Stealthy, actually. I have issues with Stealthy, but not the same ones you do. Most of what Stealthy does is make something that is normally up to the DMs discretion, and for that character it is automatic, with some limits. ie, if the guard ain't looking, or is distracted, you can dart past unseen.
The rest of what it does is take what you can normally do a step further than what is strictly possible. Ie, walking past a guard who is actively searching the area for you, in your direction, without being seen.
The problem I see in all that doesn't apply, IMO, to the others. That is, what it actually does isn't clear. At first I thought it only did what I've already always done with stealth, but...now I think I was wrong, and you can, by intent, walk right past someone. And I'm fine with that, but I'm not fine with it being unclear whether or not that is the intent.
Diplomat: it definately does more than that. If you are giving, "they can't attack even if you say something that would nromally provoke that response, and you have advantage on all interaction checks with them until they get some time and distance from you" as the result of a diplomacy check...you are houseruling that. That is...an extreme benefit to get for free.
Same with Menacing. If you are giving the Frightened condition, or a mechanically close equivalent, for a single intimidation check...that isn't something the rules should assume is a part of that skill.
The feats are entirely appropriate.