"...a guy's got to start somewhere...." Yep. That's what loading manuals are for. The Hornady manual has M1icon Rifle specific loads, but that's a fairly recent thing. Any manual's 168 grain data will do. If you want a shooting because it's fun load, use 150 grain FMJ bullets and work up a load.
The rifle was desgined to use .30 M1 ammo with a 174.5 grain bullet at 2640fps. Not .30 M1 ammo's 152 grain bullet at 2700fps. In 1940, M2 ammo's MV was raised to 2800fps to match .30 AP's 168 grain bullet ballistics.
"...Did the US ever manufacture ammo for the Garand that used a 168 gr bullet?..." .30 AP at 2800 FPS. Match bullets they weren't.
Relax. Loading for an M1 isn't rocket science. There's no magic load that shoots well out of all rifles. You have to work up the load for any rifle. It just doesn't work that way. The only really important thing is to full length resize every time and watch the case length. If you're going to use match grade bullets, weigh every powder charge too. Match quality ammo requires meticulous loading techniques.
Hodgdon gives 48.0 to 51.2 of IMR4895 and 43.0 to 47.5 of H4695 for 168 grain bullets.
The CCI primers will do nicely, since that's what you have, but you don't need magnum primers(that's all CCI 'milspec' primer sare) for either powder. Magnum primers are for lighting hard to ignite powders and for extreme cold weather shooting. They burn a bit hotter for a bit longer. Don't worry about it. They're what you have.
"...M2 ball is, however, pretty dismal as a long-range target round..." It was never made for target accuracy. It's battle ammo.