Long believed that theory highly possible, anon. Never bought the training exercise story and whatever was removed from the building looked like a hazard from handling. So a bio weapon left in a building scheduled for demolition - as I believe we discovered it was - would be a cynical means of releasing contamination into the environment without fanfare. Once the source is discovered, it’s far too late.
Same folks behind this as behind Amerithrax, “Novichok UK,” and Ebola in the Congo?
No precedent, but presumably due process rights of POTUS would extend to an analogue of double jeopardy, even though impeachment is essentially a political process.
If they try it, they’d be even more fucked than they already are. Might not stop them seeking new “offenses” to pursue, though.