What are the effects of thawing food at low temperatures?
Not being in the grocery industry, I cannot advise you on what best practices are in your industry.

However, assuming you thaw the items in your chiller at safe temperatures (40&deg;F or below) as you indicated, you do not have a safety issue.

Depending on the product, there will be damage from ice crystals forming during the freezing, which will make meats more watery and degrade their texture, change the texture of vegetables, and so on.  This is more true than commercially frozen equivilent products, because slower freezing produces larger crystals which cause more physical damage.  Commercially frozen foods are often frozen in blast chillers to minimize freezing time and ice crystal size growth because of this problem.

I cannot give you specific information on what to look for in terms of quality degradation, because it would depend on the specific item.  Think about what would happen if you froze it at home in your own freezer, and thawed it for use later.  The same type of quality change will occur.  Vegetables will tend to get limp; meats will tend to exude more juice.  Of course, the consumer deserves to know that this has occurred.

Therefore, I think your best options are 2 (disclosing that the item was frozen).  You may choose to discard the item as not of a high enough quality, but it is still safe and fit for human consumption.

It may not be practical for you to store, but you could also let the item freeze completely, and sell it at a discount frozen, allowing the customer to thaw it at their convenience.

I applaud your ethical approach in actually asking.