NOTATIO

My system: at the change bold and underlined below the staff - FL. 2 muta in PICCOLO. At the entrance of new instrument bold boxed above the staff - PICCOLO.

A a bassoon player, I have seen many clef changes . I'd put only the 3rd bar to treble clef here. There is a repeated a flat which helps the player to orientate even if the melodyline distorts.

SomeTsaikovski scores have un-condensed flutes parts when all other woodwinds are condensed...Odd.

What about: sans motour/sans vibrato; avec moteur/avec vibrato; vibrato lent; vibrato rapide? But I think English would be OK, as most of the percussion "vocabulary" is in English (mallets etc.).

That would be "col Violini" - with violins.

Might be continuation of the left hand arpeggio (with cresc.)?

It is a kind of appogiatura. Usually found in arpeggios, says Clive Brown is "Classical ... performance practice".

Your notation is OK. Most harpists will play the glissando with right hand and play the last note with left (for safety). This order may change what happens before/after, though. And they don't mind if there are rests or not -it doesn't change the way the will play. You can change the last note to h...

Might be "colla parte" - follow the soloist.

Or "mp con calore" ?