I wouldn't worry about testing for contaminants.
Feeding live or frozen is fine, one thing I've learned over the years when freezing crawfish is to put them in a large zip-lock bag and lay flat in the freezer, that way they don't freeze in a big clump making thawing easier. I've found shellfish to be much cheaper than rodents, easier(quicker) to thaw, readily available and a most excellent staple for raising generations of varanids. Plus, it's fun to watch them eat shellfish, they really know what to do with them.
Best.
Ben Aller