Serious vet responses would be appreciated... Anatomical defects in our St Bernard, and amoxicillin for dogs?

WOW ,you guys have been through the ringer! I am no vet however I have had many dogs and was a groomer for 16 yrs. I have used Amoxicillin for dogs from Chihuahuas to Bull Mastiffs, and it has always worked for infections. I have friends that cannot afford the vet prices and they have even bought penicillin for fish use to save a sick pup or kitten. I wish you all the luck in the world.