212 and 168... Lid open... Very interesting. I might have erred a little on the side of heat retention, but I think with the lid closed, we might be looking at a loss of 125 degrees per hour. That's not too terribly important, though. The bottom line is that firebrick's poor conductivity causes it to hold it's heat for a while. Thanks for taking those temps.
Re; char. I've noticed that when making Cajun roux, right before you burn it, there's a split second where it's black, but doesn't taste burnt. I think you hit that window.
For underneath the hearth, I don't want you to put yourself in a dangerous position, but if you can use a stick or something to tilt a brick up and shoot underneath it, that would be helpful. It's not hugely important, though. I have a pretty strong feeling that the bottom of the brick will be considerably higher (700+) and it would be nice to confirm that, but it isn't essential. Basically, whatever temp you can achieve underneath- that's the temp you can get on the ceiling if the hearth were removed.