Can we sue for personal injury without main piece of evidence?

A product liability case is where the product is defective or is unreasonably dangerous.  In this case, the product was defective as it had a rubber glove inside, which should not have been there under any circumstances.  What you should have done was take lots of pictures and then send the pictures of the glove to the company and keep the glove for your use at court to leverage a settlement.  Based on the letter that was written by the company apologizing for what happened you may have a leg to stand on.  Furthermore, you have witnesses that will come to court to testify as to what they saw.  Testimony is powerful evidence, especially when there is a letter from the company admitting fault.  I suggest hiring a lawyer to review all the letter and documents and filing a lawsuit.  How much to demand for settlement purposes is something to talk to your lawyer about.