I used much silent changes of fingers as it is common amongst organists.
Yes, changing the finger on a key while it is pressed is necessary here in all those places where you would otherwise tie your hand into a knot, unless you cheat and use the pedal so you can lift the hand to cope with an awkward fingering change.
Btw, is it not that this overlapping is a kind of manner they played, especially in France? I have read something about it in one book concerning baroque articulation, that they did it even it was not every time written in the score...it is interesting these improvisational aspects of baroque music..
Well, I don't know about the overlapping as a general style, but one of the improvisational aspects is that ornamentation is very much left up to the performer, and anything written in is just a hint. Typically you would do different ornamentations in repeated sections. Here, with the 8 Rondeau bars returning several times, you might be expected to play them a little differently each time.
About the bird-like ornamentation...I just did not know what it is so I ignored it:)
Sometimes you see the bird's wings flattened out, so the whole symbol looks like a plus sign. I guess this represents an instruction to play just about any ornament you like. But it's better to play something than nothing.
Also the ornament in bar 23 is unknown to me.
Perhaps it means you should play an improvised cadenza.
But seriously, I suspect it isn't "an ornament" but two, and you can choose to play one or the other. What you did sounds fine.
I think this comes back to what you said at the beginning: "I don't know how to play this", and I guess the answer is that there is no single correct way, and that you should not take the score too literally. You can do as much or as little by way of improvisation and ornamentation as you like, while remaining in character.