In theory. First of all you have to pay the amount demanded & I have never come accross anybody Spanish or other who has manage to get anything back from the Hacienda. Secondly who is going to pay for the legal & the opprtunity cost of the amount.
no you do not have to pay it first.
The only cost would be to pay for the valuations.
If you have someone act on your behalf, I assume they would not do it for free.