Henry, you remind me of the painter i seen down the shore. He takes rattle cans and some templates...starts spraying colors on top of colors...nothing really makes sense and it looks rather muddy. Then all of the sudden there is art that only someone with special skill can create. To me, you're that guy but with wood.
I agree with the three lag bolts per stud I would use 3/8" diameter. You want at least 2" penetration into the stud itself so add the thickness of the sheet rock and the french cleat to that to determine your minimum bolt length. Here is more geeky structural info than you probably want: The weakest part of the connection is the sheet rock. Do not over tighten the lag bolts it is possible to crush sheetrock. Structurally the sheet rock is a 1/2" empty space between the studs and the french cleat which means that you trying to bend the lag bolts slightly. It would be good practice to keep the lag hole in the french cleat as small in diameter as you can so that you have a tight fit where the lag bolt goes thru the cleat. Make sure that there are no threads on the part of the bolts within the cleat. Some lag bolts have "upset threads" where the threads are a larger diameter than the smooth part of the bolt - don't use those.
Remember that the top of the shelves will pull outward slightly on the french cleat and the bottom of the shelves will push in against the sheet rock. Make spacers the thickness of the clear at the bottom of the shelves to spread the load on the sheetrock. This will also keep the shelves plumb.
Enough geeky stuff.
Stud centering would also be critical if there happens to be power run beside it. Doesn't take much to have too much angle coupled with being off-center to nick the insulation. The issue may not show up for a long time ...
Don't assume a precise 16" on-center location. Sometimes there's a little twist, bow, or something in a stud. There shouldn't be but there is.
New stud finders locate more than just the steel of screws and nails. They find wood, copper, and other things in the wall. Worth the investment. You could map/mark/verify the whole wall before doing any installation.