Articulating Mount 65" led TV - 1 stud
Plan 2 (solid plywood) for the win. That kind of weight (with extra leverage) would be terrible in just drywall with a single stud. Speaking of leverage, if you can consider making the strip of ply a little taller (like, 2'), it should be a little more solid. (Admittedly, this might be overkill.)

In addition to getting the plywood into at least 2 studs, you'll probably hit something in the corner as well, which is good.

Where you have a stud behind, you might as well use a longer lag to catch both the ply and the stud. Where it's just drywall, I'd be comfortable using a stubby (like, 1-1/2") lag, but a sturdy toggle wouldn't be bad either. (Or you could put t-nuts on the back of the ply and use threaded bolts.)