"[1] We understand that we cannot close the account that has been set up in EFF’s name and that returning the donations to the individual donors would be complex and difficult."

Returning the Bitcoins would neither be complex nor difficult. For every transaction you know the sender address and the recipient address. So just return the money to the sender address: as long as the sender didn't remove his old wallet file this address will be valid.

(The only case where you can't return money to the sender address is if some type of virtual "bank" like MyBitCoin was used. In that case sender address do not correspond to the addresses of the individual users. However, the large majority of users doesn't use those banks, but runs their own clients.)