OK seems I have lost a track a bit. ( so far I was thinking ognasty already told you he sent the refund back but to uknown address )

Lets sum it.
- isoneguy sent BTC to ognasty ( from coinbase ).
- trade failed so isoneguy requested to return BTCs
- but isoneguy did not wanted to be refunded to sending address (coinbase)
- ognasty haven't send any refund yet as he has not received any signed message from sending address (which is impossible if it was coinbase)


My impresion is ognasty just strictly following his rule nr. 2 which I appreciate.


That means the mistake definitely happened on isoneguy side as he used third party walllet provider (without access to private key) not being aware of escow rules (again isoneguys mistake)  
Thus you can't blaim ognasty for strictly following his own  (public known) rules. If you did't read it it's your fault.

But this not mean there is no another way to proof isoneguy is the sender ( coinbase transaction printscreen seems pretty sufficient for this ).

So my opinion is:
@isoneguy - you should apologise to ognasty ( and all others you marked red being his alts ) as this all is mostly your fault (used address you have no private key from)
@ognasty - you should find other way (make exception) how isoneguy can proof those funds ownership without signing a message ( as it's obviously impossible for him)
or
refund the sending address (then it become fully isoneguy's problem how he deals with coinbase, but you will be clear)
... and maybe reconsider your rule nr.2 to be more user friendly ( to me it's valid and relevant rule but as you can see errors happens ) ... just to avoid similar issues in future (or you could ask signed message before receiving funds ... i know this makes things complicated but could safe you/customers lot of troubles)