Good stuff. Considering music is probably the most subjective human experience ever, I would think it would be up to the band to declare themselves mainstream or whatever they want. It's so difficult to say whether this band sold out or that band sold out. What parameters are involved and how can they apply, consistently, across the board? Impossible. I agree though on the *feeling* that some bands have "sold out" but my feeling doesn't make it so unless the band says, "yeah, we sold out. thanks for buying our shit."
Back to the question: I would doubt that Opeth consider themselves mainstream. We'd have to ask them. I also don't *personally* think they are mainstream. When I told my wife, who is constantly exposed to "mainstream" crap that I've never even heard of, I was going to see Opeth in concert, she asked me if I would be the only one there. Her jibe meant, to me, that Opeth is obviously not a mainstream band. I simply don't think the majority of music-listeners want to hear death metal.
I would guess that all of us here could compile a dynamic list of mainstreamness. At the top would be "acts" like Britney, Christina, Outkast, Etc. and near the bottom, with a low mainstream quotient would be Opeth, Rotting Christ, Etc. followed by garage bands no one can possible know. I think the middle is where it would get kinda gray.
ShroudofDusk has a good point too.....