Snakes can ingest a little bit of substrate, and it will do no harm. If they are to get any stuck in their mouth, or ingest large pieces, then you would have a problem.
As for the mice/ rats, I don't know what to tell you. I feed strictly live just because I have had nothing but problems with frozen thawed, and I hated thawing them, and some of my snakes just prefer live. I personally do not see a problem in feeding live, it is just that supervision is required.
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