TV wall mounts should always be mounted directly onto studs if the total weight is that much. If all mounting screws are going into studs, the drywall thickness is irrelevant. Drywall anchors are a great invention, but not suited for the kinds of load and forces a swivel wall mount exerts.
If you can't mount the wall mount onto studs for some reason (like, say, because the studs are spaced 24" and the mounting screws on the wall mount are <24" apart), mount a length of 2x4 lumber onto the wall and secure it onto the studs. Then mount your tv wall mount onto the 2x4. I suggest 2 strips - one for the upper and lower rows of mounting screws.
And of course read the instructions that come with the wall mount to see if they have specific recommendations for dealing with your situation.
The vast majority of TV mounts have multiple slots across their face to provide a range of choices for mounting bolts. Decent ones are fairly cheap and I would recommend getting one that gives you some choices on positioning support.
![tv mount](https://i.stack.imgur.com/sFlCt.jpg)
If you must use the one you have, drilling two holes in a solid area of the mount should not compromise the unit. You should definitely put two lag bolts in studs on the top and two on the bottom.
Mounts are designed to allow the TV to be positioned anywhere along the track, even fully to one side. the force of the TV on the mount is mostly shear force, pulling straight down. Counteracting this force is the shear strength of the bolts and the stud materials.
The effective force on the bolts and studs is based on where the weight load is in relationship to the bolts and studs. If the weight were exactly centered between the bolts and studs, the effective weight would be divided between each stud and vertical pair of bolts and be halved (43 lbs on each). If the weight were totally on one end and there were a middle set of bolts and a set of bolts on the opposite end, the weight on the middle set could be increased to the full load of the TV (86 lbs downward) and the pressure on the bolts at the opposite end of the mount would be the load of the TV, but upwards.
Any serious lag bolts 1/4" or more, and stud should have no problem handling that shear force.
If you were to have the middle bolts further away from the TV than the center of the mount, you start to get leverage forces that could multiply the strain on the bolts and studs that might compromise their ability to hold the TV in a stable position. The likely weak points would eventually be the threaded holes in the studs rather than the bolts. You could also see a pulling of the TV and its mount out of horizontal.
In sum, so long as you do not compromise the integrity of the mount and have the mount holes separated by at least half the width of the mount, you will probably be fine.
BUT, once again, I would urge the selection of a mount with a wider range of mounting choices to benefit from its full width for support.
SOME ADDITIONAL COMMENTS: The vast majority of TV mounts in general are only intended to span two studs. Additionally, if you are willing to add a bit of depth, you could attach the mount to a 1x6 board (or a strip of 3/4" plywood) using nuts, bolts and fender washers through the existing holes, and then mount the board or strip to two or even three studs with lag bolts.
SUPPLEMENT:
The Original Poster has now added information that this is a full motion mount. Because of the outward hinge aspect, these mounts almost always have one vertical pivot line in which all of the weight is concentrated. This makes the entire unit prone to horizontal twist forces and lateral tension forces (outward), not just shear forces. This also makes it more important to spread out the load, either with a new mount that has more mounting hole choices or using a bracing board that is then mounted to the studs. It also calls for deeply embedded lag bolts (at least 2" in the stud).
There are some full motion mounts that can accommodate 70 inch sets, have multiple horizontal slots and even allow some horizontal adjustment. But they are much pricier than basic mounts.
![enter image description here](https://i.stack.imgur.com/MZ1Zx.jpg)
Links and images are for illustration only and are not endorsements of goods sources.
Best Answer
I would use two or three "max brackets" with a simple wooden structure attached. Select the largest size that fits the depth you need (factor in the wood face plate thickness). I like black as they aren't so conspicuous.
Mount these with three 5/16" x 2-1/2" lag screws each, into framing. Then screw 2x4 rails to each, on edge, and a 2x8 rail across the front, using the same screws. Pilot with a 3/16" bit. Like so (side view):
Use lag screws into the horizontal rails and to attach the front plate also. Those in the front plate should be countersunk just below flush so they don't interfere with the TV mount.
If your TV mount point is low on the unit (like with my LG glass model), Use 3/4" plywood instead of the 2x8 and extend it downward 12-16". This will allow you to move the brackets up and out of sight. You might also upgrade the top rails to 2x6 to reduce the torque of the front plate on their ends. If you see too much flex, run diagonal braces up to the sides of the top rails: