In the past year I have had two test positive for heartworm. Both were on monthly Ivomec doses as a prevenative.

Where do you folks live who have Heartworm positive dogs? Pretty scary!
Just a question...have any of these dogs been on breeder administered IVOMEC? The suggested dosage for this varies all over the place. But, maybe a slightly higher dosage than is found in doctor prescribed monthly treatment is the way to go.
I treat with IVOMEC. I use .25 cc's for my 65-70 bitches and .35 cc's for my 80-90 pound dogs. Many breeders are using up to 1 cc/month.

Luckily, I have never had a heartworm positive dog.

The slow kill method is what causes heartworms to become resistant.

Do you mean the medication 'Ivermectin' or a brand name called Ivomec?

I don't know of any breeders using Ivermectin at the proper dosage that have dogs with heartworm ever.

It's my understanding that once you dilute the liquid ivomectin it doesn't have a long shelf life. Dilution errors are common too.

I don't dilute it. I have a large syringe and pull up the correct amount for each dog. I put each dose on a little piece of bread and put it atop the food in each bowl.