Is it possible to obtain expression for specific UID or currently picked instance just like IID does(Sprite(0).X)? I would like to use them.
IIRC, back in the early days, Sprite(N).X would actually look up the Sprite with the UID N and return its X position, but it got changed to index because it was more useful. I can't recollect the exact rationale now, it was too long ago, but just thinking about it now it does seem more straightforward since Sprite(0) always returns the first picked instance, Sprite(1) the second picked, etc. without needing to know any UIDs.
If you want to get expressions from an object with a specific UID you can already use 'Pick by UID', and put 'Pick all' before it if you want to also be able to pick objects which don't match the parent events.