@ Inertia, I cleaned both battery posts, and corresponding cable ends 2 weeks ago, and they are still clean (see 1st post).
Battery took a charge, took it to local parts chain, and had it load tested. It checked out good.
I removed the Battery Tender terminal from positive cable end, and re-attached both battery cables. Turned the key and she started right up.
Going to start looking for a short now.