Universae Ecclesiae says specifically, as I quoted above: A coetus fidelium (“group of the faithful”) can be said to be stabiliter existens (“existing in a stable manner”), according to the sense of art. 5 § 1 of the Motu Proprio Summorum Pontificum, when it is constituted by some people of an individual parish who, even after the publication of the Motu Proprio, come together by reason of their veneration for the Liturgy in the Usus Antiquior, and who ask that it might be celebrated in the parish church or in an oratory or chapel.
This is the definition of a stable group. A stable group is 1) some people of a parish who 2) come together to ask for the traditional mass. That’s what the document says. Here it is again, somewhat abbreviated:
A group of the faithful can be said to be existing in a stable manner * when it is constituted by some people of a parish who come together by reason of their veneration for the [traditional mass] and ask that it might be celebrated in the parish church.*
From the statements made in the original post, a stable group in the sense of Summorum Pontificum exists in her parish
Two or three people make a group. Two or three people qualify as “some people of an individual parish”. That’s all that is required
You are entirely missing the point that the bishop has no need to make arrangements because there is a willing pastor. It is true that if a small group that did not have a willing pastor started making demands for a traditional mass, then the bishop has no obligation to try to force a priest to celebrate the mass. But when there is a willing pastor, there is no need for the bishop to arrange or accommodate anything
Assuming there is a group in the parish who is asking for a traditional mass and assuming the pastor of the parish is willing to celebrate it, then the bishop has no absolutely no authority to forbid it.
The bishop has to coordinate the liturgical needs of the diocese as a whole. We do not know all the circumastances and must allow the Bishop to be the judge
As a general rule, priests are limited by Canon Law to saying one Mass, of any kind, per day, including Sundays. In many dioceses priests are already overstretched and of necessity have to be permitted to say 3 or even more Masses on a Sunday, and traveling considerable distances between those Masses too. This priest may well be one of those.
This in and of itself is an extremely undesirable state of things, regardless of how willing the priests may be to say extra Masses.
It doesn’t allow much time for the priest’s other duties - hearing confessions, taking communion to the housebound or hospitals, giving last rites to the sick. Very importantly too, it doesn’t allow for the priest to devote much time, as he should, to his own prayer life and make his Sabbath the true day of rest which it should be for everyone, including priests.
Priests are as susceptible to burnout as anyone, possibly more so, and care needs to be taken that they do not overexert themselves. Then too, they are expected to be and are usually incredIbly generous with time and effort, and often at risk of being too much so.
Now imagine a priest already saying 3 or 4 Masses on a Sunday, in who knows what far-flung parishes, who is perhaps not young or in the best of health, who goodheartedly wants to add yet a heavier burden to an already hectic workload in the study and saying of an additional Latin Mass!
Of course the bishop is rightly entitled to think first and foremost of the needs of the vast majority of his flock, who may not wish to attend nor would benefit from an additional Latin Mass. As well as the very real need for the priest not to exert himself beyond his strength and health. And so may be within his rights to say no to the extra Mass.