Are you going to run a long campaign, stretching across millennia of history that will show the inevitable downfall of Toril, or play a bunch of key moments in history, where Toril is slowly changing into Athas?
If you use the latter, you could easily explain any inconsistencies between DS and FR lore as wild myths or legends of the past. Just like Europe suffered two tremendous setbacks in society, technology, literature, sciences and philosophy (the Late Bronze Age collapse and the fall of Roman Empire), so did the kingdoms in Faerun. Candlekeep, the vast centre of knowledge was razed to the ground, after a cataclysmic event dubbed the Cleansing Wars. Historians, sages, and scholars were either too preoccupied with their own safety or instantly decimated. People stopped reading and studying, focusing more on surviving. Having lost any connection to the history of the Old Age, anything sung in taverns or around the camp is taken with a grain of salt (as the Wanderer claims). Gods living in the past? That’s a fascinating myth, fella! The Underdark? The only holes I’ve seen deep in the earth are the unlit mines of Urik and Tyr. Cormanthor Forest? Probably an older name for the Crescent Forest.
Make everything as vague and mysterious as possible; that’s what I really enjoyed with the Original setting, knowing almost nothing of the past, and doubting any fanciful or wild story.