If Vyktor didn't have a reaction to Convenia, you're probably OK. The issue with Convenia is twofold. First, as an antibiotic, it's only treatment indication is that it's to be used for skin infections. Vets tend to use it far more broadly because it's an injection. For a cat that's hard to pill, they like it. Which leads to the second issue. It's a shot that lasts for 2 weeks. If your cat has a reaction to the drug, you're stuck with a drug that's in your cat's system for 2 weeks. There's no way to prevent it doing whatever it's doing. This is the reason we encourage you to explore with your vet whether there are other options. There are people here who's cats have gotten Convenia and never had a problem. It's really not a good choice for dental issues.
The good thing with the Metacam is that Vyktor hasn't been on it for more than several hours. You can easily switch to something else. Be aware that the vet may be hesitant to give you a narcotic. Tramadol may be an alternative. Basically, you want to stay away from any of the nonsteroidal, anti-inflammatory drugs (NSAIDS), like Metacam. There isn't a great deal of research and what there is, makes you want to stay away.
Also, the vet scraping tarter off of a cat's teeth when the cat is conscious is a great way for the vet to conclude that your cat has "personality." The vet should be glad that you didn't bite him. My vet has a flyer on why
anesthesia-free cleaning is a bad thing. Gabby has problems with FORL (feline oral resorbtive lesions). They can be painful and given how good cats are at compartmentalizing pain, they only way you know this is happening is with dental xrays, a good dental exam, or if BG numbers start running higher due to the inflammation and pain associated with the condition.