I expect buy and hold and collect yield on BTC will have greater returnsover next 4-5 years than the bitcoin contango trade.
You did broadly enumerate these risks, but my biggest concern would be IB's (or really any Brokers', but particularly IB's) volatile and capricious margin and liquidation calculations. These are somewhat new instruments in a nascent and extremely volatile & fragmented market; and every IB client knows by now their slavish devotion to The Algorithm when it comes to margin and liquidation. They could monkey with margin limits with no notice, or a flash spike in the fragmented crypto exchange world could trigger a liquidation, with no recourse to you. In short, in matters of margin, I simply don't trust IB to 'get it right' or be transparent and/or reasonable. I don't know how to quantify that risk, but it's probably enough for me to steer clear for the time being.