I sympathise with your situation. I have had several bank valuations over the years in Spain – for various reasons – being sporadically active in the market and I can tell you that they have all been the most unprofessional, lazy, inaccurate and misleading multi-page documents you could encounter. Never based on like for like properties, areas or condition, – just random previous sales pulled out of a hat. I’ve never got anywhere by complaining or by producing counter arguments as the banks say their hands are tied by the Bank of Spain and they are not allowed to question the valuations. Though even so, I have always remonstrated with the bank managers, to the point of shouting and swearing, in the hope that if more people do this it may change their attitudes and get them to do something about this disgraceful farce. -Sadly the Spanish seem reluctant to cause unpleasantness when they are wronged, and accept being charged for it.
If the property is truly the best you can find and you can bully the bank into letting them give you some business, maybe a personal loan, -or credit card? might bridge the gap, if you could see your way to higher payments for a while to keep the long term mortgage interest rate down.
I would also say try to get a reduction in the purchase price, but I honestly hope they refuse as it’s terribly unfair on them.