If they want the initial collection to happen using the FI warrant exception, the purpose of the collection must have foreign intelligence as a significant purpose. So the idea they would be able to repurpose 702 for immigration cases is not only baseless, it would be unlawful
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At least per Semiannual reports, they are not all audited. https://www.emptywheel.net/2017/11/30/hype-how-fbi-decided-searching-702-content-was-the-least-intrusive-means/ … Also, NCTC is doing back door searches now too, and not disclosing its USP impact.
The selectors used under 702 must reasonably return foreign intelligence information.
Even the example of that one child abuse search is one of inadequate record-keeping on these; they didn't catch it as a search w/o face-to-face questioning. And that was by an analyst.