Maps during similar periods of Earth history have been laughably inaccurate, so we assume that Faerunian cartographers are offering "best estimate" distances.

Hard disagree on this as a gauge, they have access to magic that can answer universal truths. If a cartographer or merchant of that time period had the access to those answers they'd've been uses then too. You can cast a spell as simple as Message (with a known max distance) along with Find Familiar to measure distances as you document, and you have wizard researchers that have access to Wish (or clerics that talk to gods) for creating fixed, atomically precise, measuring tools.

I don't disagree with the rest of your post but this addition really weakens your argument because they have so many more resources in the setting.

Edit: oops this was meant to reply to one you your other replies