> Someone who wants to sell $1M USD of BTC may be happy to take $1M USDT instead for legal/tax purposes.

This sounds extremely foolish. Why would anyone intentionally wish to hold such a large amount of a token that all evidence points towards being actually worthless? (Tethers can only be used to purchase cryptocoins in Bitfinex, which is most likely insolvent -- it is not possible to redeem them for USD).

> 4) BTC price becomes stable or only moves upward and attracts more outside money and speculation. Repeat the cycle.

I think this is the most likely explanation for Bitfinex's behavior. They know that there will inevitably be a bank run when Bitcoin's prices show a sign of going down and that will unravel the scam they have been running so at this point the only thing they can do is continue to push up the price at all costs.