From Calscape: "Pinus monophylla grows in areas that receive between 8 and 18 inches of precipitation (Zouher 2001), and can grow for 350 years, but most are younger than 150 years. They are considered the most drought tolerant of all pines."
Although native to somewhat higher elevations east of the Sierra Nevada, the pines are commonly grown in lower, high desert areas, so I don't think suitability is an issue, except insofar as California's climate is rapidly changing.
It's frustrating when inexperienced growers believe "if a little is good, a lot is better." I have a neighbor who heard about the advantages of mulching and now mulches his landscape trees six inches up the bark. I've learned to ignore his yard, but in your case, you installed the trees and have a stake in the game.
It's difficult to assess your watering schedule without knowing more about the soil and surrounding conditions. You describe it as sandy — that could work to your advantage in avoiding overwatering, except for the unfortunate decision to add water-retaining amendments to the planting hole. However, that does sound like a lot of water, or at the very least, too frequent.
New trees do need extra water for the first year or more, especially in blazing weather, but it's really hard to prescribe a strict "water x gallons x times a week" without accounting for endless variables. It is never recommended to allow the roots to remain constantly wet. Four times a day never allows the soil to reoxygenate. The soil amendments can also be impeding free drainage. Did you do a drainage test before planting? Other than the amendments, did you follow best practices for spreading roots and flair depth? It sounds like you do have experience, yet, like Stone, I've got more questions than answers because there is more to the survivability of these trees besides the amount of water they're getting. But drowning them is a surefire way to kill them fast and you clearly understand that!
Unfortunately, arguing to your homeowner that a bunch of people on a gardening website in other parts of the country disagree with her watering theory probably isn't going to convince her, although it might encourage you to take a firmer stance and seek other resources. Find some local authorities — tree farm owners, UC extension services, certified arborists, water agency guidelines — and get the facts specific to your area to present to her.
Maybe send her this article. It's a good explanation of why overwatering is bad
https://www.natesnursery.com/p....
https://ucanr.edu/blogs/UCANRC...
Although based on Portland, here is a good guide
https://friendsoftrees.org/blo....
