target turntable shelf-how much weight

That's a very good idea about using the oak board as an attachment device. I may go that route some day, when I really feel like tweaking (and if I can get the screws back out of the wall--I used long stainless deck screws to make sure I hit the "meat")!

There is one way to utilize the other mounting holes on your shelf if you are worried about the load. You could screw together a wood frame, mount it to the wall studs with heavy screws, and attach the shelf to it.
I have the older Target shelf, it has different frame with more support and six mounting points on the outer sides. In my old house I mounted it near a room corner where the studs were closer than 16" and used this method.

I use one with a Garrard 401 in a heavy plinth, and I've switched out the MDF shelf for a heavy maple platform as well. The way I've ensured stability is to attach the Target steel shelf to a hardwood board (secured in all six of the connections), and then I attached the hardwood board across three studs.

I used a red oak solid plank 23" by 12" by 1/2" and was able to span three studs by starting alongside a window molding. It looks great since I stained the hardwood backing a similar color to the maple shelf and the plinth. I may not have done it if I had to make it 34" wide (i.e. the span of 16" studs x 3), but I was lucky to have the window in the right place. A 34" wide board would not have looked nearly as good.

You should be fine as long as you use a good stud finder and hit the center of the studs, and also use wall anchors for the center connections.

I just installed one. The WW1 I believe it is spec'd at 70 lbs capacity. You can install drywall anchors for the center holes and use studs for the outside screws. That's what I did and it is supporting my Marantz TT15 just fine. But that's only 15 lbs or so.

I'll probably upgrade the MDF board to a maple platform and use better spike hardware at some point, but it seems to work fine. Pretty easy to level it out all around and quite stable.

I played a record over the weekend and my toddler daughter was dancing around and running around to the music and it didn't skip a beat. This is installed on an interior stud wall--for best isolation it is best to mount on an exterior wall, if you can.