If you wanted to include them, they'd definately have to be pre-1375 DR (because Khelben's still alive)...
A possible *fix* that I've thought about incorporating into my own FR, is to suggest that the "Khelben" featured in the 1377 DR storyline of the DDTS books, was in fact Tsarra-in-the-guise-of-the-Blackstaff instead.
Actually, didn't Steven put in Blackstaff Tower that Tsarra spent most of her first few years disguised as Khelben?