Issuers of tether that is unbacked are defacto short. They thus embody your theory by design.

Is there any derivatives market in tether to allow an outsider to open/create a futures contract as a seller (thus, short) that gives them the obligation to deliver tether upon expiry?

I suspect that such matters don't work as you believe they do.

Shorting generally is done by professionals in regulated assets. Tether doesn't seem to meet that requirement. No professional will want to take on short risk in any size in an unregulated asset. Shorting, in theory, has unlimited downside risk (to negative infinity). This would actually be a factor in an unregulated market controlled by scammers, depending (in large part) on the net positions of underlying and derivatives by opposing parties.