I understand you have been doing a great amount of research and it is very possible I will not be able to tell you anything that you have not already figured out yourself. You may want to find out the exact name of the Gas that your vet used to sedate your puppy. Then you will be able to look at interactions of morphine, gas and antibiotics and their relation to puppy’s and/or Boxers in specific. Your puppy did receive three strong medications and may have had a severe adverse reaction just from her own personal sensitivities or a breed specific sensitivity. Also, there is the possibility of where exactly the injection was made and if it had caused any damage. The infection, if identified what kind (bacterial, fungal, etc) may also shed light on the various types of reactions that can take place. I’m sorry that I cannot give you a specific answer, which you and your puppy so deserve, but many of the keys to finding out this answer are held by your veterinarian and the information he has. If you can get more specific information, I may be able to tell you much more than the obvious. I hope that I have been able to help you at least a little.
sited this information about ANESTHESIA
You may want to ask your vet what kind of anesthesia will be used, as the older drug Halothane is not considered as safe as Isoflurane and other newer anesthetics.
If you have a dog prone to seizures, talk to your vet about avoiding the use of phenothiazine tranquilizers (such as acepromazine) and ketamine.
Certain breeds are thought to have problems with certain drugs, including Belgian Shepherds, Greyhounds and other Sighthounds, Brachycephalic dogs (such as Pugs), and Mastiffs, Boxers and Bull Terriers (Acepromazine), so be sure your vet is aware of this before your dog has surgery. Note that Acepromazine and possibly morphine are among the drugs that Collies and related breeds (Australian Shepherds (including miniatures), English Sheepdogs, McNabs, Old English Sheepdogs, Shetland Sheepdogs, Longhaired Whippets and Silken Windhounds) may react to. The Handbook of Veterinary Drugs says "Giant breeds, as well as greyhounds, appear quite sensitive to the clinical effects of the drug [Acepromazine], yet terrier breeds appear more resistant. Boxer dogs, on the other hand, are predisposed to hypotensive and bradycardic effects of the drug."
See Your Pet is Going to be Anesthetized... for an excellent article on anesthesia issues. For more technical info, refer to the Veterinary Anesthesia Support Group.
sites this information about Acepromazine, a tranquilizer that is often used as a preanesthetic agent and the sensitivity to Boxers.