Basically the best chew is whatever your pup wants to chew (and is safe, obviously). Dogs can definitely have individual texture preferences.
All heartworm medications in the US require a prescription. This is because of how heartworms and the medication works. Heartworm meds aren't a preventative in a traditional sense, but actually a monthly treatment to kill any developing worms while they're still tiny. When a heartworm-free dog starts taking these meds on the recommended schedule, any new heartworm larvae entering their system are killed before they're ever large enough to cause illness or damage the heart.
However, if a dog has an active heartworm infection, there are larger, mature worms literally living in the heart vessels. A dose of regular monthly heartworm medication could cause these mature worms to die en masse, causing blockages in the heart and potentially killing the dog. The process of treating an active infection like this is much more difficult - the process can take months, the dog needs to be on restricted exercise to limit strain on the hearts, and there's still a chance that the dying worms can cause deadly heart complications (but so can the living worms if they're untreated long enough).
Basically, the vet wants to make sure that they're not going to accidentally kill a heartworm-positive dog by giving it heartworm preventative, which is why it's prescription only pending a negative heartworm tests. It's an extremely quick and easy test if I recall - just a drop or two of blood on a little detector device, kind of like a pregnancy test.
Some heartworm meds also protect against fleas and ticks, but not all. Your vet can help you with what's right for your dog and the pests in your region.
