I have only passing knowledge of the laws of Mexico. Any legal action should be discussed with a legally licensed Mexican lawyer.
As a legally-constituted craftswomen's cooperative in Mexico, do we have a legal right to complain about the use of the images of our own persons and of our family members to promote the sale of products from which we receive no benefit?
As asked, it appears so. Mexico has a "right to publicity", including the right to allow or deny the use of one's image(https://en.uhthoff.com.mx/articles/publicity-and-image-rights-in-mexico/). However,
We were the principal supplier of jewelry (50K/year) to a customer who refused to raise the price 7 years (January 2000-December 2017).
This suggests the possibility of a contract. The contract can contain multiple terms, including potentially the granting of the right to use imagery that the cooperative holds(which they possibly might hold on behalf of their members). Additionally/alternatively, the permissions to use such images may have been obtained from the subjects by the photographers directly.
If not a term of the contract, such permission may possibly be revoked (though receiving payment can potentially complicate this); if part of the contract, it may still be revoked, but doing so may necessitate the termination of the contract.
Side note: Generally speaking, unless mandated by law (unlikely, especially since they are not an employer) or contract terms, the buyer is not required to raise prices. The seller may be able to refuse to sell at the old prices (unless required to do so by contract terms).
That business was sold in 2018 to a lady who used to be the stock girl; she claims no knowledge of any accounting before her tenure. We want to "close the books", since we dont have the complete record; only the customer kept track of the differences so as to round everything off in their favor.
The buyer's records should be irrelevant to your accounting*. The buyer could sell the jewelry, give it away, or destroy it, and this should be irrelevant to In general, the cooperative gives the buyer jewelry and receives either money or debt in return. In either case, normally one receives a record recording of the receipt jewelry from the buyer (e.g. "On DD/MM/YYYY, received 5 bracelets" or whatever)**.
*Unless the "buyer" is not actually buying the jewelry, but rather selling it on consignment, which is a completely different relationship and scenario.
**Thus, even in the case of debt, this should not matter, as the cooperative should have its own, independent, records of its delivered property, funds, debts and invoices pending, for tax purposes if nothing else.
What makes this different, is that these people assure their customers that they are doing a good deed for traditional third-world artisans. Otherwise, probably, they would not purchase. Can these people somehow be legally held to the principles they espouse to their customers?
Generally, this would be dependent on the laws of where the borrower sells to their own customers. Usually, one is only held to account to act in accordance with laws and obligations(such as those established by contract); however, there are frequently "false advertising laws" prohibiting lies in advertising products. However, depending on the advertising, this may not be false. "We buy directly form a co-operative of Mexican craftswomen" would be true, for example.