The only reason you can freely trade between BW and B2W2 is because they're part of the same generation.
True, that could be said of FRLG and RSE as well, but considering the RSE Pokémon already exist during the time of FRLG, I'd say they're concomitant.
That's an interesting argument, but RSE can't be after DPP because of all the new moves and evolutions that are in DPP but aren't in RSE. Unless, of course, they remake RSE. I might be inclined to think RSE happens in-between FRLG and HGSS, but never between the two Unova games.
With that said, Hoenn and (all) other regions more than likely existed with their Pokemon before we visit them in the games. That, however, does not mean the big events(what we have to go through playing the games) happen together. There is pretty much nothing solid that gives Gen III a specific spot is what I'm saying.
Other than that, the others seem to have a better way to judge where they are.
In GSC the new Johto Pokémon are referred to as "newly found", although FRLG screwed up the logic by introducing Johto Pokémon in the Sevii Islands.
If RSE happened during 5th gen, there would certainly be Pokémon not indigenous to Hoenn that were introduced later in the series, such as Sinnoh and Unova Pokémon. Again, a remake can fix that, but the fact FRLG, which is the very first game chronologically, can trade with RSE without the machine being labeled a "time machine" as it was in GSC, to me may not be conclusive proof, but it points in the direction of FRLG and RSE being simultaneous. If RSE was after FRLG, there would be explicit sayings that it happened after, much like with all Pokémon sequels to date.
Also there are the Orbs in HGSS which allow you to harness the power of the Weather Trio, it's pretty unlikely Mr. Pokémon would be in possession of such items allowing a trainer to summon those Pokémon before Team Magma/Aqua even could. Finally, the Jade Orb, to call Rayquaza, didn't even exist in RSE, it could have been discovered later on.
And remember that in Emerald, which is the true canon for Hoenn (as B2W2 demonstrates), the protagonist does NOT catch any of the Weather Trio during the storyline, making them available for Ethan/Lyra in HGSS.
Very slim chance. I haven't played B2W2 but I've seen videos of those NPCs and IMO you're seeing only what you want to see here. BW also makes mention of the Go-Goggles. It was probably just a nod to Hoenn (like pretty much everything Hoenn-related in B2W2).
Sincerely though, I think Cyrus was infinitely more dangerous than Archie/Maxie, so his actions would leave a much bigger impression, enough to be known by Team Plasma. The Team Plasma Grunts also don't make mention of Team Rocket, whose actions are way more "down to earth" and, therefore, more likely to be believed and known by ordinary people.
Well I'm pretty sure Bulbapedia put it there for a reason. The reason is that, to date, the place most plausible to fit RSE is during the events of FRLG. There's way more proof of that than there is proof of Hoenn happening between BW and B2W2 (in fact, there's no proof of the latter).