Is there any proof that anyone has ever successfully redeemed USDT from Tether?

Yes, there are even people on HN that have reported successful redemption of tether for USD. You can ask people directly. It's not like this is a rare thing that never happens. If people weren't able to do so you would expect people to actually make claims aginst it.

I have not seen anyone on HN ever claim to have exchanged the necessary 100K USDT for $99k or 1M+ USDT for $1M+ with Tether's parent company.

Look back to one of the Tether submissions and you will find someone who says that their company was able to do it.