Are the feats and weed from stage 1 useable, despite not coming from the 2 boosters you use in the event?

Nope.

Seald is sealed and, as far as I know, no instructions from WK have said that the feats can be used during the sealed portion (like the instructions that said Surfer and Co. could be used during the CoG events).

If you're bringing in feats from outside your boosters, it ain't sealed.

I remember they were saying that the prizes for weeks 1-4 weren't useable until weeks 5-9, but maybe they were just saying that the special object from week four could be used in week five...