Thank you all for your assistance. I never realized how handy the Magic Wand is (I'm new to Paint .NET). My issues are fixed, danke.
Oh, and I suppose this is 'by design', but when I erase some pixels from a top layer, pixels from a lower layer show through (which is the behavior I was hoping for.) Kinda neat.
Yea, that may sound silly to the experts out there, but I'm very new to all of this, so I'm having some 'oh duh' moments.