OP - I think this is mainly about them. I am guessing they feel guilty about the previous person being recruited and employed under their watch. They are now completely paranoid about it happening again. As others have mentioned they might also feel threatened by somebody as experienced as you esp if they feel they didn't do a great job of managing the dodgy ex-employee situ.
Do you feel what they are asking of you in terms of reporting is unreasonable? And it sounds like they are not asking for the same level of detail of feedback/reporting from the other AM? If you feel that they are focusing just on you and tarnish you with the ex-employees bad brush, then I really would consider trying to find some type of official support within the charity either HR or branch support? I am sure you can have an off the record conversation initially?
Or do you know any other M/AMs in other branches who you could reach out to for support?
I appreciate that following your difficult personal time recently (well done on leaving an abusive relationship, it is such a feat
) I can understand you not wanting to rock the boat...but they aren't playing fair. It sounds like you are over-delivering and it is still not good enough.
The position of your post is that the trustees they are doing their job and you need to meet their requests. However, they may well have good intentions etc but they could actually be poor trustees (the email from them you allude to sounds very unprofessional and unhelpful) and by raising this you might be able to help the charity sort out a branch which isn't quite working IFSWIM.
And it isn't ideal that the other AM is both an employee & a trustee - do you mean trustee of the shop's charity? It is allowed but I wonder if they give enough thought to the conflict of interest this brings - you should be equals in the shop as two AMs but how can she be if she also sits on the board and makes strategic decisions...esp about staff??? I know it is a small local charity but trustees are meant to provide strategic oversight, they sound overly involved and not really clear of their role. Is there a chair? Are they any good?
If you don't want to go the the charity. I'd be tempted to have a meeting with them/a representative of them and outline that you are doing what you can, that you are reporting back/providing feedback in line with the other AM - assuming you are - and that they just need to trust you. If they want to increase the reporting & feedback (and you feel it unnecessary) push back explaining it will impact on the amount of work you are able to do and that if they ask it of you, then they need to ask it of the other AM as they have to treat employees equally and fairly.
If you are concerned about such a meeting send an email first. This is never ideal, but I took that approach once when I just couldn't summon up the courage to the say what needed to be said. We then met and it was ok. It does mean you can spend time getting the wording & tone right.
As much as they don't trust you, they probably hate the thought of having to go through recruiting again, so I think the power is probably on your side - although I appreciate you need the money/want the job so don't want to risk loosing it.
And going forward, as Silver says they need to elect one of them to be your line manager, not a committee. And ensure that the other AM is line managed too. And therefore any feedback about performance needs to be channelled through them, not all of them sending you bizarre emails when the fancy takes them.
You have been through such a lot OP. Try to summon up the strength to accept that you are working to the best of your ability and this is their issue not yours. Push back where possible "Have you asked AM for that too? What will you do with that information?".
Finally, as silver says, are there any volunteers that were working when the dodgy employee was that are still volunteering? Are there issues with them still being there? He may not have 'acted alone' etc....