I think your approach re. the Chamber Choir is sound -- you are, at very least, giving yourself a non-toxic space in which to build your musical vision, alongside the issue of managing and, hopefully, bringing along the disgruntled members of the long-dormant former choir (dare we compare them to a long-dormant volcano at last erupting?) eventually through patient care.

If I'm reading this right, it sounds like a lot of this is happening in the semi-darkness: the whispered complaints, the priest's private confiding of her liabilities in you in a private forum, the hijacking of a closed, choir-only event to air grievances.

I would take good notes of their complaints, their themes, quietly note them without making too much noise about them. Take note of words of appreciation you have received from the wider community. Then, I would have a public meeting (make sure it's recorded, by the way -- "for the benefit of those who can't attend") -- frame it as an informational session on liturgical music and the growth the program is experiencing. Make sure the priest is there, invite the whole parish to attend if they like. Let them know that there will be free food and a Q&A period. Make sure important community members who have expressed support for you are definitely coming to this.

Then, one of two things happens:

(1) The cronies attack you in full view of the parish community. Everyone transparently sees it for what it is and it is both on-record and recorded.

You know the themes of their attacks in advance and robustly defend your programming and procedural positions in front of the entire parish. Emphasize the fact that, whatever was done in the deep past, you inherited essentially a dormant program and few working procedures or processes.

Community members have the opportunity to publicly express support for you and what you are doing.

The pastor can see the vitriol, if it comes out, and formally deal with them on a disciplinary / pastoral level (in private afterwards) based on what he sees, setting appropriate boundaries.

(2) The cronies do not show up or remain silent at the meeting.

If they try to poison the well under cover of darkness again, call them out on the fact that you gave open account to the parish of your choices and they did not avail themselves of that opportunity.

Tell them that this is what is now being done, you have the support of the pastor, and they are free to leave if they wish, but X,Y,Z are non-negotiable, they are how things will be done for as long as you are in charge, for articulate reasons.

To the extent that you can also indicate that the Chamber Choir is doing just fine, and (politely) that they are non-essential -- the parish's musical life will go on without them -- it can create the right perspective... this is not existential, they are non-essential, and you are extending an offer to them to participate to give them an opportunity to serve, not because you have need of them, nor in any way that gives them power or invites a power trip.

The Golden Rule: If your articulate reasons are sound to your best judgment and that of your pastor, based on best practice and liturgical norms, you need only to inform, not to convince your naysayers.

They can remain unconvinced if they like, but: "It is my job to run this program. I find it most manageable / makes most sense to do it in this way, and I am satisfied with the results I am getting, many of your peers are also content with it, and the pastor supports this." -- that is perfectly sufficient. No more needs to be said. No further comment needs to be taken -- if you're feeling very nice, you can say, "Thanks for the input, I'll consider it, but ultimately I'm going to do what helps me run the program efficiently and effectively and build the systems I want to see in place in the long term," or even, "I'm glad that worked so well for you, but I prefer to do it this way for [reason] and that's the direction we're going to go for now. I regularly evaluate my processes and will modify things if it still seems necessary to me even after people have had time to get used to this way of doing things."

You have (delegated) authority. You can speak authoritatively. Informing, setting boundaries, and simply stating facts calmly and objectively. Do not take it personal, or make it personal.

Thank you, NihilNominis, for taking the time to write such a thoughtful, thorough, multi-dimensional, and helpful reply. Thank you, Musicguy 57, for your support. NihilNominis, touched on something imporant here - the choir appreciation "reset" brunch was not public facing. I learned that the old choir director refused the offered retirement reception but her retirement and past service was acknowledged through the bulletin and from the Ambo. Your music informational gathering idea does hold several advantages, including the public "reset"/clarity of roles, as well as allowing the parish to learn about the new musical direction in an open context, countering her effort to project an image of still having one foot in the music ministry after retirement, (while also passively aggressively refusing to follow up on her offer to sub at the organ when I'm ill). This could go a long way to counter the undermining, triangulation, whispering after mass, etc. I think it's a lovely idea, and what better way for the parish to officially "meet the Director" while being welcoming towards questions, and encouraging wider parish musical participation. Have you ever done anything like this? Thank again!