Is it fallacious to appeal to scientific authority when the current consensus is precisely what is in contention? If yes, is this an example of Scientism? If no, why not?
No. No. Category error. Unless you’re publishing in the peer-reviewed literature, the scientific consensus wins. If you don’t like the scientific consensus and you’re not publishing—nobody cares.
For example: ivermectin does not cure or prevent covid. If you argue this with your doctor, they will invoke the scientific consensus and not write you a prescription. If you argue this with your insurance company, they will likewise cite the scientific consensus and not reimburse you. And if you argue this with your local tack and feed store, they’ll ask to see a picture of your horse and, when that isn’t forthcoming, cite the scientific consensus and kick you to the curb. The fact that you really, really want to challenge the scientific consensus doesn’t make the consensus invalid.
Scientism is the belief that scientific inquiry is the only valid form of epistemology. As best I can tell, no one believes this. The term is mostly used to slur your opponents. Arguing that you ought to be allowed to ingest horse dewormer because Jesus told you to is not an effective argument, nor is labelling your opponents with “scientism” because they don't accept divine revelation as a useful guide to pharmacology.
If you want to challenge the scientific consensus effectively, go to grad school.