Imo this remix doesn't sound like something they really put much effort in making. It's so simply remixed. Probably other remixes will be like this too, including Hoenn ones. I guess they don't want those tracks to sound better than in games they belong to, and are saving better remixes for R/S remakes.
Well Kanto was in gen IV which was also in the Ds so not much change can really be expected, Hoenn is proably going to be the most different seeing how Sinnoh and Johto's remixes will proably end up like Kanto's (all already had DS music).
I think the under water tunnel is to go from Vibrank to Castella. (Gym 2 to Gym 3) Since there is no visible bridge that connects that city to Castella.
As for Lancousa, its not that they are into old stuff. They are just stuck in a Superstition. They don't have to be in B2W2 cause well 2 years pass and someone could have ballsed up and stayed out all night for multiple nights (if Lancousa isn't the Frozen Town from the Trailer)
I think that the Tournament in the Anime may be in Lancousa town if not in an Anime town seeing as that tournament could be the end of that part of the journey.