I bought a celeriac bulb the other day, it had the celery stick and
leaves still attached. They are very very green and wilted, but I
thought I might be able to put them in stews and soups. I've frozen
them. But then I wondered, are they safe? Could they be poisonious.

No worries about poisonous, and I don't think there are big issues
with flavor/bitterness/etc. At least one of my seed catalogs lists
their celeriac as "celery/celeriac".