I completely disagree with you. It looks nothing like Zekrom nor Reshiram and seems the most unrealistic choice to be in their Trio.
And no, Second Gen didn't have any other Legends to use that weren't event. Using it doesn't hurt the argument unless the person arguing against doesn't know anything about Pokemon or the history. Suicune also was part of the Beast Trio and isn't even considered to be equal to the Legendary Bird DUO of G/S
Only two games that can be easily compared to are R/S/E series where it was written in before Emerald, and This set is still argued today to be a Duo with a Master, but has insufficient evidence to back it up, rather than a Trio with a self ruling Master. And D/P/Pt which pulled a R/B/Y and based the third Game off of the Anime/Movies.
IF the dragon can relate to them, it will be in Black and White's history for it. So far, the Dragon/Ice has been shown but hasn't been given any info on relation to them. Only that you find it near a lake that suddenly freezes.
Its ability not relating, lower stat which ALL TRIOS ARE EQUAL with exception to Rayquaza because he is a Master, AND not having a perfectly matching attack list to the other two, this all says that fan speculation has very low chance for success.
What I'm trying to say is I don't believe its a Trio. As for Mascots, any NON Event Legend can be the Mascot. We don't even know what the name of the third game would be and would have to wait 2 years before anything could be hinted.