Hi Brian Newcastle
Not sure what the differences are. I use frontline with my two. Good questions. I apply mine when I know my dogs wont be in contact with water ie a Monday.
Your vet will let you know re vaccination.
I would be inclined to do a heartworm test. I am not sure re dermatitis does your dog require weekly injections for dermadex. If this is the case then go with what your vet suggests.
Good on you for caring I hope it all goes well.
Ness