I think you are on the right track. If you linger on the appoggiaturas, they may start to sound right. They are very expressive. The German edition calls these Sonatines. Are they called Sonates in the French edition? You are about right on the dates of ornamentation. Mostly, it is a question of style and function. Is it a grace note or an appoggiatura? Well, it depends on what is happening harmonically. If it makes no difference at all, it may be a grace note. They seem to be most common in triple-metre rondos. In a slow movement, they are almost certainly on the beat, taking their rhythmic value from the following note. You may not have heard them played that way before, but it should be right. I have a three-movement sonata by Naderman that clearly indicates the classical style of ornamentation, on the beat. In that third movement, I would play them on the beat and accent the dissonant upper note and give it a tenuto and diminuendo as it resolves on the third note.
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