I wouldn’t bother with standoffs unless you want the tv farther from the wall. The bolts for the TV mount to the plywood are fine going into the drywall. I’d personally just get some shorter lag bolts though if you can. It’s really no big deal, but the bolt really only has to be long enough to reach through the mount and the plywood.

I’ve done this several times with a machine bolt having the rounded end facing the wall and the nut end securing the mount. Works very well.

Leave out the stand-offs, if your TV is less than 50#'s I wouldn't even bother with the plywood. Mount the bracket with 2 lags into a stud then use heavy drywall anchors rated for 50-100# each into your drywall. The single stud will support the weight while the anchors will help they will serve to prevent sagging downwards. If it's more than 50#, 1/2" plywood is More than enough as a backer, or just grab a 1*6 and cut to needed length. Whatever is cheaper or on hand will suffice. Mount that to studs with some construction screws then use included hardware from TV mount to install centered on the board/plywood. Make sure that the screws going onto your board (if used) have threads in the wood and not smooth shafted when fully installed. If they threads are not towards the head of the screw you won't have any holding strength so in this case just go to the hardware store and get some that will work properly, going into the drywall isn't necessary but won't hurt overall, don't assume it to be structural though (this is why you need threads of screws into the board, not smooth shaft)

The drywall anchor idea is interesting. We are also dodging a mysterious and solid 6" wide something in the wall. There is a laundry room above this wall but nothing lines up in an obvious manner so say it's pipe or just air vent or what. My stud finder reports solid for 6 inches left/right between 2 studs gapped at 24" inches

That could be a lot of things. With a laundry room over top you can have a drain line as well as water lines lined up going down through a plumbing wall, as well as electrical. Honestly I hate stud finders personally. You can always take a drill bit or screw and run it in slowly and see if it hits something, if not use the anchors. Or just try to mount where there's nothing in that area. The other option is cut it open and see what's actually there. If you use anchors you can get ones with screws that are short enough to not hit anything after testing the depth with something like a drywall screw or just a small drill bit. Make a mark on the bit or screw so you know the depth that it's going to run in when utilizing the mount and if you don't hit anything in that depth you're good to use it in that location. If you can't find a spot that satisfactory just mounted on a piece of board and move on

I've been doing something similar for years.

I used 1/2" Plywood, and then used 1/4-20 T-Nuts on the back side of the plywood, matching with the holes for the mounting plate, then used 1/4-20 x 1/2" Truss Head Machine Screws (truss head screws have a larger head than regular, so don't need a washer) to go into the T-Nuts.

Then I used the 2-1/2" (I think) lag bolts that came with the bracket to go into the Plywood and studs where they cleared the bracket on the outside, and/or 2-1/2" drywall screws in places that the lag bolts would get in the way of the mount.

This is a heavier articulating mount that pulls out, so perhaps I went a bit overkill, but I have used this same pieces in now 3 apartments, and just have to adjust where the screws go to match the studs and center the display accordingly.

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I have two boards like this, this picture is of a wider board I have that also has another mount to the right of my 49" display that is for a 27" display that is portrait for my desk setup, but the principles the same. You can see the countersunk holes from the drywall screws from a previous apartment.

Image:

https://app.box.com/s/i7zbb9adb5gsvbz93rk790g2xk5tjz6a

And so you do this without stand-offs?

Yeah