Stacked stone retaining wall problems after 10 years

We purchased a house 4.5 years ago where the only updates the owners did was to terrace and design the yard. Amazing! However, as we are trying to make changes to the design, we've learned (and have observed from issues with the walls) that the stacked stone walls were not built appropriately. Examples include insufficient anchoring concrete for walls higher than 3', gaps between stone with nothing to secure them, small base stones supporting a lot of weight. 
We asked the landscape architects (licensed) to come look at the work, and they refused. They deny having any copies of the work, as it was done nearly 10 years ago at this point. Previous owners don't claim to have any documentation but they did include the price of the work in the sales paperwork. 

The person we'd like to work with brought their masonry person who says all the walls need rebuilding. This is an easily $50k project before we do anything else. Our original budget was much less than that, as that's a lot to invest when there are other critical things we need for our home still. 

We'd prefer not to approach this litigiously, but we are also at a loss. We've contacted our own home owner's insurance, as we are concerned about structural stability (walls are already pitched toward the house), and they said it's the architect's business insurance that would be responsible. 
I would appreciate insight into whether anyone else has dealt with this or the appropriate and less contentious (and ideally least costly!) approach. Are we incorrect in expecting the work (originally >$100k!!!!!!) to be sound?

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Attorney here (although not a real estate attorney).  This seems like the kind of issue that should have been discovered and discussed during the home purchase.  You should review the contract for the purchase and see if you waived the possibility of raising any claims for items that were either disclosed or were easily discovered (i.e. they didn't hide the poorly built walls in some way, which it sounds like they did not).  I realize in the Bay Area housing market people are purchasing homes without good inspections and sometimes with no inspections at all.   But if the walls are almost 10 years old and you've owned the home for over four years, I think you either have to live with it, or pay for the project.   

First of all, I'd get a couple more estimates for the repair/rebuild work. In my experience design/build contractors ALWAYS want to re-do from scratch. Not so much because they want the money but because they think their design is better, and they're not that interested in tweaking someone else's design.

Secondly, a retaining wall that is higher than 3 feet needs to be permitted, at least in Berkeley. So you might want to check with your city to see if there is a permit on file. In my experience, licensed contractors are unwilling to risk their license doing that kind of thing without a permit.

We put in a stacked-rock retaining wall in our hilly backyard about 20 years ago that is still strong and beautiful and holding the hill back. Over the years, small filler stones have occasionally fallen out, and you can definitely see gaps here and there between stones. But it is overall well-built and sturdy and shows no signs of failing. The foundation for it is below ground level so I have no idea how large the base stones are but I assume our contractor did the math and built it well. So I would say that gaps are not necessarily a problem. But like I said, you should get a couple more people to look at it.