I'm sorry but I'm seeing an issue where you knew what your players wanted... knew you did not have the knowledge base to give it to them, but decided to run it anyway... and surprise they were disappointed. I mean you know what limit you want... one book, why not establish that as the continuity for your Realms and if they don't like it or want more... well then choose not to run it since you aren't going to make them happy (and more importantly as evidenced by your posts, you're not going to have fun running it).
EDIT: I'm sorry but I believe the campaign world and it's boundariesi/makeup are up to the DM and this illustrates a big reason i lean in that direction. He or she is the one ultimately running it, tracking it, keeping the continuity going, etc. So yeah if the players are asking for something you don't want to do or can't it's ok to limit or even deny what they are asking for.
I don't think you understand the scope of the problem.
The Realms canon includes everything published by WotC (and TSR), which includes campaign guides. But it also includes novels (and there are scores of those) from many authors. It also includes modules produced over the past 20+ years.
A good portion of those older materials (1E and 2E) are out of print. The newer (3.0 and 3.5E) sourcebooks, of which there are dozens and all hardbacks, often pull snippets from the older materials but also include regional and historical updates. Now that we're into 5E, which doesn't have a campaign guide yet, you're lucky to find even 3.5E books - and WotC doesn't plan to make a campaign guide or even think about one until at least 2016 or later.
More than that, Realms canon includes anything Ed Greenwood has written about the Realms. This includes thousands of answers to fans, over decades, at the Candlekeep website - and these are dispersed through hundreds and hundreds of webpages at that site.
There are also scores of mini-articles, brief glances at various people and places in the Realms, many of which used to be hosted at WotC - and some still are, if the web links are still alive.
When someone says, "there's too much to catch up on" for new DMs, or even DMs that have been away from it for an edition or two, they really aren't kidding. It's impossible to know what your players have read and therefore expect you to know because all of those materials have never been collated into a single source or browsable location.
There's the FR wiki, but it contains a fraction of what's been deemed canon. And it's not always correct or thorough enough.
Beyond all of that, FR contains a HUGE amount of silly cheese, hundreds of overpowered epic characters, crazy gods wandering around like NPCs, and a host of other things you might want to edit out beforehand.
For example, most recently, the Realms overgod got mad at the main pantheon gods, decided to teach them a lesson because they were naughty, and in short order this led to the death of several well-liked gods, the magical nuking of the world, several outer planes being mashed together, another "twin" world crashing into the Realms and swapping out or obliterating entire nations. They also jumped the timeline ahead 100 years and didn't fill in any details of that period at all. This is now all being "undone" (at least in part), with the worlds moving apart and some gods/nations returning, but no one knows the extent or scope of those changes because there's no new campaign guide, only novels. And those novels don't provide but a few details.
