Since it appears we have verified cases where people received a USD check as a settlement, when they
- Requested a BTC refund
- Didn't request a refund at all (through the form they issued)

For such people, the check is thus just an unsollicited donation from Hashfast.
I was wondering, how long do you have before the check expires? I think in france you have 1 year to cash it in...

So just keep the check, resolve the dispute by getting a full BTC refund as promised in the ToS you agreed to.
And when you have the BTC back, cash the check in to cover for the hassle!