I'm not sure about that. I was just adding to your explanation as I have seen trills written out like that in RCM books. Probably Sidokar will now chime in with a detailed explanation of the historical evolution of the trill.
I can chime in, if someone is interested.
Thanks much for your offer. Your knowledge would be much appreciated
Thank you for asking. Here are a few elements.
The baroque period is a very long one over 150 years. In that period there has been a number of different ways to play the trill. There are numerous ways to vary it and there are different trills, short or long, prepared, accelerating, with a stop, with a termination, ….
There is no consistent and unique way to start the trill in the baroque music. It depends on the exact period, country and composer.
Though it is true that toward the end of the baroque, typically end of the 17th century and first half of 18th, there is a consistent trend to start with the upper note. Which is what CPE Bach exposed in his treatise. That was largely under the influence of French keyboard practice, but there are a number of documented cases, though not the majority, where starting the trill with the main note is better or using a prep note played in anticipation of the beat.
Mozart was trained by his father. The influence of CPE Bach is not really obvious, unlike Haydn who we know was following his direction. For example, CPE Bach does use very often the turned trill sign (chevron and turn underneath) which Haydn also uses occasionally but which Mozart never used.
So, in his early years, the best is to look at what Leopold Mozart explained in his Violinschule published in 1756 and which had a considerable success. Leopold Mozart is following a lot of the Italien ornamental practice like the one of Tartini or Geminiani.
So, in general Mozart style owns a great deal to the Italian cantabile, unlike CPE BACH or other german composers who have a more harmonic heritage from Bach father. Mozart has a very vocal and melodic way of writing, like Chopin later on.
The trills and other ornaments must be decided based on that melodic continuity. In general, for the very young Mozart (5 to 8 years) the trill would start on the upper note. The music being fairly simple, that is a rather reliable rule, even if there are probably some exceptions.
Later on, Mozart has been influenced by his time, so certainly went away from his father teaching and evolved his practice according to the practice of musicians around him.
The trend to start the trill with the main note seems to pick up around circa 1775, where a number of italien treatises come back to the old Italian vocal legacy. It then starts to generalize in other countries but most experts would agree that both ways coexisted for a long time. Even Chopin who followed Clementi approach still uses trills which can start with the upper note or the main note depending on the case, when on the other hand Czerny trills almost always start on the main note.
For Mozart, we dont have any written documents that describe directly his point of view, so we are limited to study what most common treatises of the time say as well as his compositions where occasionally he would explicitely write the trill.
The outcome is that it is fair to consider that the basic short trill can start on the upper note or the main note. The upper note would probably be the most common case, but maybe not as frequently as with CPE Bach. Main note trill makes sense in a number of situations when there are appogiaturas (small notes), or descending stepwise motion. Like for Chopin it is essential to keep the cantabile melodic line rather than disrupt it with a dissonance. Cadential trills are usually started on the upper auxilliary, assuming it is the dissonant note. For long trills if nothing is indicated, I could start them on the main note with possibly a stop and a termination.