So I figured out the gridlines. That part's good. The lasso-global-lower-temperature thing didn't work out so very well, because all the uneven spots within the selection area stayed uneven and still needed to be sort of pixel-painted individually, and of course the level of detail isn't great though my editing resolution's quite high, so I can't fully repair the area. There's a stupid half-moon shape of spots left and I can't change them without altering the surrounding climate too...so I'm leaving them alone. Fixing the climate globally would work but I just don't have the stamina. I don't think I could do a great job having to adjust ALL of that anyway.

What FT seems to need is a way to dial in the settings one wants in selected area. Just type in the temp/rainfall/elevation settings to be applied to the whole selection. Right now the only tools allow you to type in the DEGREE OF CHANGE which isn't helpful if you need to even out an uneven area. Fixing anything takes much longer this way than it needs to.