Both of which are pretty big issues in the scenario we're talking about. When Tether crashes it will take down a significant chunk of the crypto world with it.

I wouldn't be surprised if congestion is extremely high on ETH if USDT collapses, but I'd be shocked if the chain started missing blocks.

Do you think that USDC would depeg as well?

If the bid and offer are like $2 - $.50, do you consider that a "depeg" or not? I think liquidity will disappear, transaction fees will go to the moon, and there won't really be a meaningful "price" because trades won't be happening.