I'm sorry that Thumper didn't get to have his teeth cleaned, but it sounds like a good solution for Fonzi.
Like I'm sure that you have been, I've been frustrated that the vet didn't see anything, either during a regular exam, or when he went in.
I wouldn't have another procedure for it, either, but there's a root cause to everything, and it would have been nice if they had found it.
As for Preparation H...it would be great if it did help.
When I wanted something to heal on a dog, I'd often mix 3 or 4 things together before applying. Usually, it was a bit of vitamin E, a touch of aloe vera gel, and an antibiotic gel or cream - if needed, depending on the problem. You could mix the aloe, E, and CBD, for example.
If you do get aloe, look for one that doesn't have a lot of other ingredients in it. The type that people take internally tends to have more brands with less added to them.
Also, adding a drop of lavender oil can be helpful. But, it has to be actual lavender essential oil. If you can't find it easily, I have an extra bottle (I use it in my wet wipes, and such, so always keep an extra.), just let me know. You're welcome to it.
Always do a patch test for everything. Oils, aloe, etc. Like people, sometimes dogs react to things.
It sounds like you got a good head start on cooking. I hope that it all came out delicious!
Love All Life, Thank You For Posting! :)