Who is doing testing of these powders for toxins/heavy metals/purity/etc etc and what does that cost? Can you trust the results of said places performing these tests?

It costs from $600 to $1000 if one wants full suite of tests. There are at least two popular third party testing labs. Since testing is expensive, buyers pool together to share costs. In large groups, one ends up spending $10.

If you pool together doesn't this increase the risk of you being prosecuted for dealing these compounds?

Yes, if you sell to others; no, if you join with others who buy from the same batch from the same source.