Couple of questions for the more experienced?

It's mis-information that giving a dog with heartworms Ivomec will kill it. Ivomec only kills microfilaria, not adult heartworms and it will not matter how long it's been. I've got an old dog positive for heartworms that I've been giving Ivomec for years to keep from developing any more adults. Come to think of it, the lifecycle of the adults has probably passed and he's probably clear anyhow.

Also adult heartworms don't mature and develop inside a single dog. Simply put, microfilaria must be picked up by a mosquito and placed inside another dog for the lifecycle to be completed and adult worms to develop.

As for pup shots, the mother's milk has nothing to do with protection. The only antibodies a pup receives are passed from the mothers blood (from the placenta) and from an antibody-rich milk called colostrum received during the first 36-48 hours of life (and ONLY if the mother was properly vaccinated prior to whelping). The antibodies received will last several weeks and as a rule injectible vaccines will usually be started around week 6. There's a period of time there (generally called the window of susceptability) that the mothers passed antibodies aren't strong enough to offer protection, but still strong enough to not allow a proper vaccination. Since there's no way of knowing exactly when that time is and it varries from litter to litter and pup to pup, that's why pups are usually vaccinated starting around six weeks, then again every three weeks for four rounds until the vaccination hopefully will become effective.

Although kinda rare, it IS possible despite the mother being up to date, and injections being given that the pups can become infected during that window.