Just a few thoughts regarding the USDT:

I] - I suppose everybody read this by now: https://www.coindesk.com/tether-claims-30-million-stable-token-stolen-attacker/
II] - Tether is worth 673 M USD at the current price it's 81219 BTC - https://coinmarketcap.com/currencies/tether/
III] - Tether is "audited" by somebody reputable who actually said it was not an audit but something like "an opinion" (last time I heard this was during the defense of rating agencies after the 2008 CDO/CDS fraud).
IV] - For those who don't remember - when the fraud is big enough even auditors are happy to get their hands dirty - lookup Arthur Andersen and Enron scandal (60+ billion was lost almost overnight in a fraud in year 2000 money ~ easily 600+ billion in today's money).

I don't like baseless panic and FUD but I think there is something smelly about USDT like the OP said - I'm not sure what or how but it doesn't add up.

Is there a way to independently check how many USDT is actually out there?

Is it possible that somehow there is fake fiat aka USDT (wow double fake) buying crypto and artificially pushing the price up? My biggest concern is that there might be 10x or 100x more USDT than they actually report.