In a score containing players with multiple instruments the instructions for players when switching to a different instrument (e.g. “to vibraphone”) should be hidden in the score layout, and the respective key signature changes should appear just ahead of the new instrument entrance. How?
About your first question, you can highlight them and open the bottom panel in Engrave Mode and tick the appropriate checkmarks (and just use an empty space in Custom Text) but make sure to set this property only locally in the score.
I posted a workaround to have the key sig change before the entrance a couple of years ago in this thread. Basically you need to use independent key sigs so the key sigs between the switching instruments are identical and then switch to the correct one at the entrance. It’s a bit tricky for percussion though. You can use a chord symbol region (hide chords if any) to force the change to occur later. This will make the score look correct but will break the multibar rest in the part. There’s not a great solution to this issue yet.
So I guess there is no option to provide this globally?
And then there is a (percussion-) clef change following which I cannot get rid of. Scaling to 1% or greying out does no longer work for clefs! What can I do?