Private lending is a  business not a how nice a person can I be contest 

Many of the properties require significant repairs, have clouds on title, will involve contested and difficult evictions even after the auction, etc. 

I considered trying to contest it with my own comps, but everything that I have heard about that indicates that it would be a waste of time.

As a start I went online and found these estimates for a tax confirmation suit:Average total duration: Uncontested: 6–9 months Contested: 9–18+ months, depending on disputes or appeals.2.

There's no contest.

You cant just "take ownership without contest," especially if the seller fights it.

A lis pendens indicates that a foreclosure lawsuit has been filed, marking the property as legally contested and likely in preforeclosure.

I have gotten taxes reduced by contesting the value of the B-C properties. 

True, as Borja and Erwin suggested, banks are much more flexible in the US than in Spain but what matters way more in wealth building is the level of interest rates and there is no contest there.Normally, banks worldwide only lend to their own citizens, there are citizens and there are Spanish and American banks that will lend to non-residents.A Spanish resident could get an interest rate as low as 2.5% and a non-resident would pay 2.9%.

The person has threatened to contest the quit claim if I try to sell, a credible threat given their history.