what do you do when a metamour is really nice to your mutual partner but mean to you (in subtle ways) and your partner can’t see it, not because they really think you’re lying or anything, but they really like the metamour and the metamour says bad stuff behind your back and everyone denies it but you’re pretty sure they’re (consciously or unconsciously) trying to drive a wedge between you and the partner?

This is a really tough situation. On the one hand, it’s important for your partner to have your back. I would recommend talking to them in a private, relaxed setting about specific things the metamour did that you felt were subtly mean. I have had to do this with my partner - when a mutual acquaintance was being disrespectful to me but in ways hard for others to see, I pointed out later what had happened. My partner hadn’t realized anything at the time, but once I called his attention to the pattern, he acknowledged that it was going on and he tries to support me more around that person.

If your partner denies it - if they say you’re being over sensitive, of the metamour didn’t mean it that way, etc. then you have a choice to make. You can continue fighting for yourself, trying to get people to see that you’re being undermined and hurt. The pros of this plan is that you’re standing up for yourself. The cons is that it can make it look to others like you’re the one with the problem, you’re the one trying to drive wedges and say negative things about the metamour. This approach has a high probability of creating tension, which you have to decide whether you’re comfortable with.

Or, you could take the “high road” and ignore it. If you’re right that the metamour is being nasty, it will become obvious to the people in your social circle soon enough. If you stay neutral and sweet and polite, and this person is trying to make you look bad, you will cause them to fail by not reacting, and either they’ll stop or they’ll escalate the behavior to a point where it’s obvious to everyone. If they somehow succeed - if people in your life start seeing you negatively or treating you poorly - then you know those people are not friends who have your back.

If this person starts actually driving a wedge between you and your partner - if your partner becomes more critical of you, or spends less time with you, or starts being disrespectful - then you’ll have some clear, actionable complaints to bring up. My advice, which I know will be easy to give but very hard to actually take, is to try and ignore these suspicions for now. If things do get socially bad for you, you’ll have evidence you can address; if you’re wrong and this person is just socially clumsy or grating to you, you don’t risk being seen as the problem whose behavior started strife with someone else.