hm, they arent short and they have a defined open and close price which supports your intuition about legal structure.

their market makers and trading desks carry big locate risks - the mms are short the price until they locate the coins.

something akin to a short squeeze is a fork and that could definitely impqct black rock. they have discretion over which fork the trust adopts. if they adopt the wrong fork according to the network they will be long the wrong thing and short the right thing. even still this would just hurt though a reduction of aum not a loss of collateral.

there then is the big issue, loss of collateral. i do not like their custodial concentration at coinbase. its irresponsible to not diversify secure implementations. like keeping all cash at one bank.