Apologies if this is already answered somewhere.

I searched and couldn’t find anything relevant. Most maps I set up with dynamic lighting are combination room/corridor places where I draw in all the walls and doors, and then place dynamic lights where I want them. However . . . if I want something like a huge Manor House, which the players explore in daylight, I don’t seem to be able to give the entire a map a global ambient level of light and yet retain the visual blockers that walls and doors provide. A global ambient lights up everything and let’s my players magically see the entire map. That cannot be the intent, right? Walls should still block line-of-sight, surely. So what am I doing wrong. If anyone can help it would be very much appreciated.

Try going into the Page Settings > Dynamic Lighting tab > enable Daylight Mode I believe that is the effect you're looking for. 

I think the issue Sionel is raising is a "mixed mode" map with both indoor and outdoor elements. The global illumination settings are great for all-outdoor maps, but do not work well for mixed maps.  If that is correct, then what I did is to define some invisible tokens that emit light, make them the default tokens for bare-bones NPCs, and use them on my mixed maps to provide outdoor-only lighting. 

Walls (Dynamic Lighting Barriers) should still block line of sight even when using Daylight Mode, although the light level that you set up with Daylight Mode will illuminate everything, including the "interior" of the building. Are you possibly using Explorer Mode? That will leave an areas already seen by a player token visible to that player even after they leave that area. Also, have you tried viewing your map using a player account? As a GM, even when you log into your game "as player", you will still be able to see a lot of things that are only visible to the GM. The best way to test a new map is by creating a dummy account then inviting that account to your game.

Thanks for the feedback, folks. I'll be back in the editor this evening and will poke all of this to see if I can get it to behave. It FEELS like the thing I'm looking to do should be possible, certainly based on your comments - a low-light across the entire map, avoiding the need for a million light placement points, allowing LOS to be blocked by doors and walls. I'll update as I give it a stab later. Thanks

It sounds like what you want is low lighting everywhere. On the Dynamic Lighting tab you can turn on Daylight Mode and set the percentage to reflect the amount of light you want. Walls should still block vision…