Assuming remakes are canon, which they most likely are:
R/S/E occurs before HG/SS (confirmed by Steven referencing the Hoenn protagonists)
HG/SS occurs concurrently with D/P/Pt (Red Gyarados news report and various other hints)
D/P/Pt occurs before B/W (confirmed by Cynthia referencing the Sinnoh protagonists)
B/W occurs before B2/W2 (direct sequels)
Therefore, R/S/E occurs before B2/W2. As for the Wallace argument, he didn't appear as a gym leader either.