The answer to all 3 question: it depends. (yes, not so helpful).
Warning: Silkscreen clipped by solder mask
gerber export dialog offers a extra checkbox to delete soldermask layer from silkscreen output
some pcb-manufacturers offer to do this for you (so aks your manufacturer)
so you decide:
do it manually and move manually all silkcreen away from soldermask-areas
rely on gerber-export and use that checkbox
rely on pcb-manufacturer (this is my solution)
Warning: Silkscreen clipped by board edge
depends on pcb-manufacturer, but most manufacturers just ignore silkscreen outside from the board area
this means you are probably loosing some silkscreen information → you have to decide:
lost silkscreen printing (outside from board area) is important → correct the silkscreen position (and therefore remove the warning)
lost silkscreen printing (outside from board area) is not important → just ignore the DRC warning
Warning: Silkscreen overlap. If I’m happy for the overlap to occur, can I just leave this?
Yes. If you think you still can read the overlapping silkscreen, just leave it.
I personally have switched off this DRC-check (board setup–>Designrules–>Violation severity)