Reading through the lawsuit against Tether detailing its long history of moving money all over the place it reads like a series of moving money from one institution to the next as the institution grows uneasy with having Bitcoin money on its account. So they are constantly moving it all over the place until someone gets tired of them. Every second and third tier money transmitter starts blocking them, they're caught in an ancient system whose gatekeepers are trying to keep them out.

I would not be surprised that they don't have 100% reserves but their biggest problem is having a place to put the money. In the end, they will exhaust all resources. The best thing they could do is do a proof of burn. Where they literally have a neutral third party with cameras verify that they burn the dollars that come in (though illegal) and replace each one with a USDT.