You know, I've been following this thread but there's something I don't think either side has actually said (perhaps I missed it?)
What exactly should be considered "reasonable" information that a DM should know about the Realms...
That varies widely.
The first FR game I ever ran in was very minimalist in what the DM knew. The DM didn't even have any core campaign setting material, this was the 2e era of the late 90's so the excellent 3e FRCS hadn't been published yet, and our FLGS didn't have the 2e-era FR box set in stock. So, it was run based on what the DM had pieced together from playing in a previous FR campaign under a different DM (which was pretty loose with canon), playing Pool of Radiance on the computer years ago, reading a couple of Realms articles in Dragon, and a few novels (Elminster: Making of a Mage, and a couple of Drizzt ones I think), and owning copies of Faiths & Avatars and Volo's Guide to All Things Magical.
So, from these scattered sources he ran a FR game that was just fine since none of the other players had ever played the Realms and only vaguely knew of it as another D&D setting. If you're playing to generic D&D players that don't know the realms, or novice players who don't know D&D at all, nor the Realms from novels, you can be as minimalist as you want.
If you've got a group of hardcore fans that are the aforementioned "canon lawyers" then you'd better be one yourself, at least if you want to make them happy. The good news is that I think they are pretty rare, all things given. Yeah, they're out there (and we've even seen them in this thread), but I think my initial assertion in the OP that they are fairly rare (but memorable) seems to have been upheld by this thread.
I guess it comes down to, just make sure you know as much about the setting as the consensus of your players. For most groups, read through the FRCS and if you're going to be spending a lot of time in one area read up in detail on that area and consider getting any region-specific texts for that area if you want to be thorough. Honestly, I'd consider everything outside the core box sets and core campaign setting books for each edition (assuming you're running in the normal time depicted with that edition) as the essentials, and all other suppliments, novels, sourcebooks, Dragon articles ect. as optional.