Interesting question !

What should you immediately do? [1 pt]

Ease the jib and point winward, in order to eliminate forward pressure on
the mast

What steps can you take to prevent the loss of
the whole rig? [1 pt]

Clip an halyard on the stern. You won't get a lot of tension from it, as it
would also put a lot of dangerous compression on the spar and hardware.
You'd have about twice the compression you have with backstay, if you load
the halyard the same amount.

What can you use to replace the backstay long enough
to get home? [1 pt]

Pull on the main sheet and vang.

What else can be done to pull the mast aft to keep
tension on the rig? [1 pt]

I can't think of anything else