I feel very sorry for you, but without sight of all the relevant paperwork and full details I can't offer much more assistance. Also, I am probably not qualified to advise too much; you may be better off asking Maria de Castro (via PM). Meanwhile, for what it's worth:
I'm assuming you did not have legal representation at the time of the sale? (Otherwise, you would have gone back to whoever that was, rather than approach ABC). This is a shame, as they would have been able to chase the buyer for the 211 - which by law had to be submitted within 30 days of the sale, and the retained funds paid to Hacienda. Failure to do so would have incurred a fine for the buyer - not you. This is NOT your responsibility! The buyer "retains" the 3%, and is 100% responsible for paying it to Hacienda - that's the whole point of a "retention" at the time of sale!
Are you sure the fine you had to pay was for late submission of the 211? Are you sure you were fully up to date with your non-resident taxes at the time of the sale?
Once the retention has been submitted (with modelo 211), the seller has 3 months in which to file the 210 and claim any of the retention due back (or pay any additional CGT due). So it's a little odd that your 210 was not filed until about 18 months after the sale.
I assume that the copy of the 210 you have does in fact show you are due a refund? You did sell at a loss, presumably? You are certain you didn't owe any non-resident property tax at the time? I only ask, because I can't understand why after almost 4 years ABC are still giving you vague brush-offs - not to mention the €350 you paid for them to help with this, which in itself seems rather extortionate for something that's normally quite routine.
I can't help feeling this may be a lost cause. Sorry to be negative, but it may be best under the circumstances to write off your loss and move on.