I understand the point, but their musicianship must be up to par. It’s hard to find suitably qualified singers and organists from any background.

Catholics will surely make the same mistakes, and it was a substitution. Mistakes are more likely in a new place, with a different priest, a new pace…I can’t say if they are excusable and if they are to what degree for professional organists, but I loathe to be too harsh.

The quality needed must always be excellent, but what that means differs. Improvisational skills are a must, but improvisation is used differently in the Novus Ordo and in the TLM. You must have the ability to accompany hymns in a typical parish. Ideally, you can give incipits and accompany chant using Gregorian notation. But that doesn’t mean all skills are used at all Masses or in the same way. I can imagine that the being thrown off at the Gloria and the Mysterium fidei on Sunday if one is used to playing for a Saturday evening Mass; does she usually accompany those? Was there any feedback on the quality of the music played?

A well-crafted handout they can keep for reference and follow when playing is also a great thing to have. No-they don't need 12 pages. One should be enough.

I completely agree.
In the past I've made various versions for when we (actually manage to) have an organist volunteer to play at any of the EF Masses at a small area church. Various, based on Mass Ordinary and Liturgical Season, etc.
Similar sheets, but with chant, were made for celebrant(s) to have on hand at the altar. (I'm sure something similar could easily be made for OF Masses, where the DoM knows what their seasonal Ordinary is, and could have just a laminated sheet available)

For example:

Your priest's sentiments and expectations are laudable, but they need to be tempered by some understanding of the emotions and individual competence of any sub organists who walk, often lamb-like, into a new worship environment.

Few can deliver a flawless performance of their duties in a new church. They don't know how fast or slow this and that are sung. They are not attuned to the ritual pace and speech patterns, which are different in every parish. They can only use their own musical judgment, based upon their own level of competence, in doing their best to lead a congregation in worship. A sub deserves some understanding and lee-way.

Nor is this problem solved by relying on only Catholic organists. The same obstacles and adaptive problems will apply regardless of one's faith. No two congregations are alike. They don't sing alike. They don't sing the same music at the same speed. They respond differently to the organ. Etc, etc. It is unrealistic to expect any organist to make an A+ - though many do.

I might add that it is quite possible for a non-Catholic who knows Catholic liturgy to play a service more beautifully than a Catholic, particularly if he is Episcopalian - not always, but quite possible.

Further, e'en though a given sub played flawlessly, he or she may have played differently than 'our organist' and is, therefore (a non sequitur!), assumed to be 'not as good' as 'our organist'. These sorts of (probably unavoidable) sentiments are to be expected, though they may (and, often, do) represent the very opposite of reality.

Your priest, and many others, would be better served by insisting on a certain level of musical competency and liturgical sense - attributes which 'being Catholic' does not guarantee.

My pastor gets upset when a substitute isn't up to his standard, but can be gently reminded that his preferences don't always mean someone wasn't good or even wasn't competent. A lot of times a "I will give him/her your feedback and that will be fixed next time" is enough to get him to back down from what would be a similar decree to your Rector's.

I have a very good working relationship with my pastor and a very small geographical area from which to pull substitutes, so usually I can inoculate things a bit by saying before the liturgy "I have a substitute playing/singing today - they are very competent but this being their first time here, a few minor mishaps are bound to happen". Oftentimes afterward when I ask for feedback, he'll say the person was "fine" or, "other than a slow Gloria, they did great".

...late with the memorial acclamation and slow with the Gloria.

Are these parts pertaining to the organist where you are? I've sometimes seen an associate pastor frantically nodding to the organ, afraid to stand up on his own initiative after the Epistle.

I must second the idea of having an experienced Catholic on stand by to provide helpful reminders when a non-Catholic is the organist for a single liturgy. Obviously the hope is the organist will know what to do, but just in case someone has a dear in the headlights moment it is helpful to have someone that can lean over and say "play the Gloria..."

When I have a non-Catholic visiting musician for the first time, I try to give them a binder of everything that happens during the Mass, with their music inserted in the proper place. Then there is no doubt about cues and the whole event is less intimidating. This is a lot of prep work but everybody benefits and you'll find your subs become more confident and personally grateful.

For an organist, I may give precise tempo instructions in advance.

All of the Mass ordinary settings and responsorial psalms we use are plainsong. So usually, when I have to be away, we don't even need a sub. The pastor does not mind the whole Mass -- hymns included -- being a cappella, led by one or two cantors.

About 'prompters' and such.

While I mostly agree, I think that if the DoM is away, someone should be designated as "in charge," so to speak. Not that they get to change things around, but to keep everything running smoothly, as is the norm at that parish.
Things shouldn't fall apart just because one person (whatever their role) is missing. Visiting musicians shouldn't be offended by someone's attempt to keep them in the loop.

The trifecta. Catholic, competent and respectful.

I'm so tempted to make a Bill Clinton "safe, legal, and rare" joke. Just not sure which is which.

As I have mentioned in other posts, we have our cantors positioned next to the organ, both in the church organ loft and in one corner of the parish hall. We have a number of local substitutes, some Catholic, some not, most having substituted at Catholic Liturgy regularly. We put our most experienced cantor with the sub to help everything flow. If the cantor is a block away, at the "ambo", according to regulations (gag!) it is of no help to the organist. I hate cantors up in front, waving arms or not. the music emanates from where the music emanates. Period.