Okay understand I have to as questions because I have learned not to assume anything.
Based on the photos that I have seen that I think are real Spanish markers (4) I can only have a general idea on what I would do if I was in your situation. It is really hard to say without closer photos or more photos of markers or monuments. I already have strong hunches about things and I am usually correct about them. But that alone is not enough for most logical and sane people to go on.
However you are in the area and you have the scanner, so you have really nothing to loose. Learn and scan.
There is really no point in going on about following monuments if you already have turtles and you know where they are. Sure it would be nice to see inscriptions on the markers or drill holes for the mine shrine but...
Turtles are usually your key to the Kingdom. At least that is what I have found.
Here is the thing, pozos are capped and buried, usually about 3 feet under the ground. With your gizmo it should have no problem detecting the void under the overburden. Providing you are on top of it or very close to it.
Mine shafts are also covered. But with a heck of a lot more material.
Way more than 3 to 5 feet!
Gee I hope you like to dig
Personally I like to go after smaller caches in the area. They are easier to find and recover. But none of your photos show resting place monuments or fishing, watering areas.
If you find your mine your logistical problems have just begun. Finding is easy, recovery is the bitch.
I will try to help you if I can but without more to go on I am somewhat limited.
Do not give away your location in any way on this site. Close up photos are better as wide angle can reveal location details.
Ask any questions and I will try to provide insight.