If things are truly this bad where the accompaniment is severely distracting, I would say something. I would give exact details as you have listed here so that your MD can figure out what the next steps should be
There’s at least one lady who sits across the aisle from me who is always video recording the Mass. A lot of parishioners video record the Mass and put parts of it up on Facebook.) is going to be a challenge for me.
I tend to lean to the side of being charitable.
Your original title asks if you should talk to the priest - I would hazard that he is aware of the situation because he was there! :D
I am genuinely surprised this is permitted!
The mistakes you noted are very normal problems for an amateur who doesn't play for the liturgy on a regular basis. While it sounds like her keyboard skills are not sufficient, I don't think an angry letter would help anything here.
Was your music director away on vacation? Does he work 51 out of 52 weeks in a year? Was he gone for a personal reason like a family funeral? One bad Sunday is not too high a price for having a good music director on a regular basis.
Try saying this, "We're glad to have you back. We really missed you!"
Also, singing loudly out of sync with both (poor) accompaniment and choir is just as distracting to those around you as the poor music in the first place.
This is why I hate parishes having only the one regular musician:
Odds are, the associate priest isn't an idiot and probably said something in private to the pastor (even if only to laugh about it privately) so I doubt they are totally unaware, but I reiterate that I would be grateful to receive a (charitable) note or conversation from someone who actually knew what they were talking about. All the more-so if it was one of my choir members with whom I have a personal relationship.
Simple missal chants, or one or two very easy hymns a cappella (Holy God We Praise Thy Name, sort of things) would have been very possible.
Simple mistakes were made, apart from playing poorly. For instance, you don't do embellishments if you cannot play the real notes. It's always better to drop things out and keep going at a steady pace, than to be haltering in your playing, and riddling it with wrong notes. (I wonder if this person was trying to make up the part because they couldn't really play the music?) In any case, it needs dealing with, whether that's coaching or no longer permitting them to play.
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