Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that Duxie has been diagnosed with heartworms and I understand that you want to support her as best you can so she can go through the treatment successfully and minimize side effects.
It sounds like your veterinarian has an excellent plan in place to minimize side effects and Duxie's body's reaction to the dying heartworms.
The ability of her immune system to fight the infection won't play much of a part in her treatment. What we want to do is minimize the chances of over-reaction and inflammation which could lead to blood clots forming.
Keeping her quiet is the best way to minimize the chances of pieces of dead worms forming an embolus and causing organ or lung damage. The more active she is and the more her blood pressure and heart rate rise the more danger she is in.
Feeding her a high quality food is ideal as well.
Omega-3 fatty acids are fish oil products that act as natural anti-inflammatories and anti-coagulants in the body. I highly recommend supplementing them now and during the treatment phase. 3V by DVM or Derm Caps ES are good brand name products. Use the high end of the dosing schedule for your pup's weight, which means if she is between doses choose the higher one. I recommend a dose based upon the EPA portion (eicosapentanoic acid) of the supplement as if we do that the rest of the supplement will be properly balanced. Give her 20mg of EPA per pound of body weight per day. For example an 80 pound dog could take 1600mg of EPA per day.
Best of luck with your girl, please let me know if you have any further questions.