There is no registry showing which individuals or entities own bitcoin or the quantity of bitcoin that is owned by any particular person or entity. It is possible, and in fact, reasonably likely, that a small group of early bitcoin adopters hold a significant proportion of the bitcoin that has been thus far created. There are no regulations in place that would prevent a large holder of bitcoin from selling their bitcoin, which could depress the price of bitcoin and have an adverse effect on an investment in the Funds which do not take a short position in bitcoin futures contracts.
There's a lot of ins and outs which can get confusing in arbitrage trading. We won't go into the technicals of why futures contracts trade at a premium to spot price. You can read a full explanation here. If it's not of interest to you, all you need to know is that there's a tendency, the further out in time the futures contract expires, for the premium to spot to be higher and higher in nominal percentage terms.