I am sorry to hear about your frog, and hope whatever happens goes down in the least painful way possible for both of you
I'm personally more comfortable with the idea of injections than I am with freezing... Just a personal thing, I don't know anything about them other than what I've read on this and other forums.
Just wondering... How would you go about giving an injection to a frog? It seems like it could be painful or (comparatively) slow if done wrong..
Also, does anyone know if the clove oil works well on other reptiles, or is it just frogs/fish because their systems absorb it so much more quickly?
I wouldn't. They could interact within an animal's body and potentially result in a slower or more painful death. That might not be the case with these particular drugs, but it's better not to take the risk. Any one of them should do the job quickly, so I would see no reason to mix them.
Someone who knows more about it could give better advice, but from an uninformed POV mixing meds like that is generally a no-no.