A Land Far Away

http://forgottenrealms.wikia.com/wiki/Ibrandul

So I understand that this god is worshipped in calimshan, and died in the Time of Troubles. However Shar now masquerades as Ibrandul to the followers unknown to them.

Questions:

1) Is the masquerade known at this point by the majority of the population of faerun? If so, have the temples and faith been abandoned?

2) If worship continues, would these people be considered 'false faith' and therefore a character on this path select 'no deity'?

3) Would calimshan adventurers still exist that follow Ibrandul? (Given that adventurers are generally not 'common' exactly, at least later on)

4) If a character meets a worshipper of Ibrandul, is it possible to demonstrate that their patron god is actually dead?

My take on it is that Ibrandul is still worshiped but that his worshipers don't know he is dead and that Shar is masquerading as him. I expect no one will feel the need to investigate if they keep getting the feedback and spells from "him" that they expect.

ALFA breaks from FR canon the year that ALFA2 went live, and doesn't necessarily follow major events therein (e.g. Amaunator has not placed a permanently-shining sun over Elversult, but it's 1380 DR -- presumably he won't ascend in 1385 either, because people aren't lining up to run an Amaunator thing and we don't likely have the resources or desire to do the spellplague).

Our current canon has not included any events which reveal Ibrandul's death, so the state represented in Faiths & Pantheons / The Player's Guide to Faerûn is probably a safe bet. Though with a caveat that also answers your fourth question-- it's possible that DMs here will run events to change that.

Now there's an idea for source of inspiration for DM plots: major events from canon the past decade or so.

Isn't that a significant part of why we're playing FR anyway, because there's at least some tie to canon FR and famous events/names, etc? Although I'm pretty sure Spellplague/4e stuff is fairly universally despised...