If its not enough money to spend a few hours on, then I'm not sure its enough money to complain about either.
Where? Wherever you are, assuming in the US? I'm sure they could motion for dismissal under their TOS but at that point they will have probably taken note and fixed your issue.
Summon whom? You file suit against the corporate entity, this is how most lawsuits against corporations work. They get a notice and its up to them to either show up or not.
So if they ignore it and you get a default judgement, you can then use the judgement to collect from their bank since I doubt they are structured to hide money across international lines. Or you can just sell the judgement to a collections agency and let them deal with collecting it. Either way, if they ignore it you will likely get your money back. (Again this all assumes both you and Coinbase are in the US, if either of those isn't true then its possible for them to ignore it and have no issues)
