Hi There,
You really shouldn't feel guilty about putting the flea medication on the mother cat, it should go on the back of the neck, i don't think it would have killed her, i would have to agree with your husband, thinking that she is ok somewhere and just wondered off, have you checked with your neighbors perhaps one of them have taken her in. You could also try putting an ad in your local newspaper under lost and found, maybe if you have a picture of her post them around your neighborhood. We have several cats all indoor cats and one had got out and we looked for days, we went door to door looking, i even called the police dept trying to report him being lost, they did take a descripion of the cat, i think they thought i was tapped in the head, but we did find him 4 days later and he was just across the street hiding and scared. so don't give up hope, I hope you find her soon.
Emilysmom