Do not use drywall anchors to hang a TV that large on a swivel mount. If it was a mount that didn't have motion, I might say that's acceptable, but not with a swivel mount.
You will need to use wood in some form to add the necessary strength.
Plywood can be attached to the wall studs using 3-1/2" lag bolts with fender washers. To provide a clean finish, the drywall can be cutout and the opening filled with the plywood. Then tape and mud the joints. Sand the surface smooth, prime, and paint. The plywood will be barely noticeable if done right. Be sure to match the plywood thickness to the drywall thickness.
If you do not want to remove any drywall, the plywood can be mounted directly over the drywall. You will need to use at least a 4" lag bolt in this case to account for the extra depth of the drywall. The plywood can be finished with a routed edge, sanded, primed, and painted for a finished look, if desired.
A final option for a truly hidden mount would be to open the wall by removing the drywall. Then install 2"x4" crossbracing between the studs at the correct locations for your TV mount. Replace the drywall, tape and mud the joints, sand, prime, and paint. The bracing is now hidden in the wall and you can attach the mount to the bracing.
If the drywall was hung on top of plywood with nails or screws there's a decent chance those won't align with the studs, which is why your magnetic studfinder could lead you astray.
The plywood has to be anchored to studs, and those should be 16" on-center. So you only have to find one as a reference point. As an alternative to opening enough of the wall to probe for a stud you can use the fact that the plywood anchors are magnetic and roughly 16" on center, combined with the existing holes that failed to find a stud, to map out likely stud locations.
Best Answer
Monkey hooks are great.
Modify the hook using wire snippers. It leaves a tiny hole in the wall and they hold up to 50 pounds. Very very solid for heavy things. I trust these much more than screw+anchor - if I can get these to work for the application. They wont work for everything (like blind housing that has to screw in) but for anything to "hang" they work.