I think you answered this question yourself, already.
Your question implied that the seller and its agent represented the house as a renovated house. Then inspections revealed that there are underlying expensive problems to major systems such as the foundation that were ignored during the, ah, "air quotes": '"renovations'" ahem.
Your own experience with that seller is the best evidence you have to go on. When you look at renovated "flipper" homes assume that the renovation may have been cosmetic only and proceed with that prejudice, until convinced otherwise.
It would be wise to look closely at any renovated home to see if it is cosmetic only, or if the expensive basics such as foundation, HVAC, plumbing, electric, roof, windows, doors, appliances and other systems have been rehabbed, or if they needed it.
It is possible Guardian purchased the home at an auction, thus they do not know what, if anything, is wrong. What turned up in 4 inspections you conducted? Were the inspections from certified inspectors? If you are satisfied with the inspections, get a home warranty and move in. If it doesn't feel right, don't do it. Unfortunately, you have invested some money, but it could be cheaper in the long run and allow you to sleep at night.
Hope it works out for you.
REALTORÂ® - lic #01923240
Certified Public Accountant (CPA) - lic #103885
Hammer Real Estate Group
I would have your agent contact their legal hotline through CAR about whether the seller can be compelled to provide all disclosures from the previous seller. There may be a form or something. But you may also want to contact an attorney and understand your risk if you buy a home, where you suspect something else might be going on with the property, and you aren't made aware. You need to assess your risk.
CAR (CA Association of Realtors) may have a standard contract form that helps document the circumstances, but your own attorney would be needed to understand your full legal risk and circumstances. It's too big a gamble to go in without checking, if this is what you think.