I recognise many of these Goldcar stories, and my experience in Jerez this year was similar - rude and churlish staff, unreasonable fuel policy (although it is true that Holiday Autos do set that out in their small print) and hard sell on the additional insurance cover. At least the queue was not too bad since Jerez is a small airport - there was a massive queue at Malaga the previous year.
I declined the additional insurance because I had purchased a car hire excess annual cover policy (over the internet at home). This appears to be a money saver - you pay just £35 per annum and it covers the excess for all your rentals that year. Except that I have found that it doesn't cover everything.
I too had to hand over a credit card and it was swiped to the tune of some €600. Unfortunately we had a mishap with the car keys and a replacement was needed. I also had to fork out €120 for a taxi to collect the new keys.
My Questor policy is underwritten by White Horse Insurance Ireland Ltd. Whilst they have paid up for the charges related to the keys, they refuse to cover the cost of the taxi.
So - don't think that the annual excess cover provides total peace of mind. In effect, it is not an effective substitute for the hire firm's own additional cover (which would have cost €60 in this case). So, questionable whether this is really a money saving approach.
In future, I will be looking for a better quality rental firm, and taking out their full insurance option. In fact we did this in Spain with Hertz in 2012 and the experience was relatively problem free (yes, I dented the car then as well - don't have much luck with hire cars!)
Back to Goldcar, I felt that they were positively crowing over the fact that things had gone wrong and - 'oh dear, you don't have the insurance'. The same offhand staff were unhelpful and unsympathetic in their handling of the situation. I felt that the reason for their annoyance on the insurance front was that they probably get some personal gain from selling it.
I'm thinking of hiring a tank next time....