Walls – Using a full motion single stud wall mount for the 49” Sony 900f

After reading through the product info and the assembly instructions, I'd say the 99lbs refers to the max load of the unit itself. For one thing, they don't know what you're attaching to so they couldn't spec out a max load for the units effective bearing weight. And two, the shear load of a lag into a stud through the little metal brackets supplied would be quite a bit more than 99lbs per connection so it has to refer to the box itself. As far as your responsibility goes for the installation, I would want to see you catch at least two studs just to be safe and then use a drywall anchor like these in the remaining two locations as per the instructions. That way your unit will be pulled tight to the wall evenly across the length but the studs will still be bearing the bulk of the load. As far as being able to tell the condition of your framing, if you can tighten down a screw to the point where it pulls everything snug without stripping out, that's the best you can do without stripping the drywall and having a look. If you feel confident you're hitting the studs but they keep stripping out, either your screws are too short, you're piloting too much, or your framing is seriously compromised and you should definitely have a look.

Toggles and other heavy weight drywall anchors can be suitable for TV mounts when the mount is flush. Or toggles in to metal studs. Since the load is mostly shear and not pull-out, this is fairly safe. (Provided the drywall is thick enough, the anchors are installed correctly, the wall is properly supported, and lots of other considerations too numerous to go in to here.)

However, adjustable, articulating or arm mounts should never be used on just drywall or metal studs. No matter what anchor you use, drywall and metal studs can not handle the pull out force applied when a TV is extended or tilted away from the wall.

So either get a flush mount, or open the drywall and install appropriate wooden studs.