Just a quick progress update with screenshot. I have the 4-texture painting/blending working, a modifiable brush and opacity that raises/lowers/smooths/planes/balances terrain, a more cohesive button theme (for the buttons that have icons so far...) and an undo stack. Major hurdles left include prop and trigger additions, and it's irking me that I can't see the brush radius by default, so I need to add a brush-perimeter indicator. Otherwise it's close to "done enough" for more game development!
(Click to enlarge)
I do need to add: the textures seen here are placeholder and belong to their respective artists (thanks google images). Now that I have them painting properly I can work on my own assets to replace them.