I tihnk it is fairly straight forward regarding their insurance and getting your own Hire Car Excess insurance.
Everything except the wheels, windscreeen, mirrors, lost keys, undercarriage and in some cases breakdown recovery is covered in the car hire cost, those things just mentioned are not (and are the most likely to be damaged during your hire period). If you want to safeguard yourself against these things you can buy insurance from the car hire company at the time of collecting your car. The price will be high, extortionate in fact, but it is your choice to take it or not.
If you decline because you have your own insurance, or just because you think it is a rip off, then you MUST leave a 600 euro deposit (or whatever the cost is). You cannot negotiate out of this, even if you have your own annual policy you must still pay this deposit.
Then, should something happen the car hire company will deduct costs from your deposit. If you have your own excess insurance you would then have to claim against that back in your home country.
Buying your own insurance does not remove your requirement to pay the deposit, you must still put your card down no matter what - the car hire company does not want to get involved in a scenario where they are reliant on you claiming on your insurance to get their car fixed, and why should they? They could not run a business like that.
I had always taked the hire company insurance and had absolutely no problem with returning the cars but last year I did buy an annual policy in the UK and when I hired through Goldcar (who have always been really good, I might add) I also returned the car and my deposit was returned without any problem whatsoever in just a day or two.
These things are just a fact of hiring a car in Spain.