Mark: outstanding (as in: not yet known)

I would go for “pending” (or, longer, “still pending”).

I agree, using outstanding here is quite ambiguous.

I would use Awaiting mark or Pending mark - they are both clear and direct. (You can alternatively substitute mark with grade.)

I think something such as 'Awaiting Results' would be more appropriate.

To be received, or to be decided?

This fits into a pattern of common phrases: to be confirmed, to be announced, to be decided, which are standard enough that in many institutions they are given just as tbc, tba, tbd. To be received is less standard than these, but should be well-understood since it clearly invokes this well-known template.