I am a bit unsure about the slam fire thing. The bolt gets home and makes a last 1/8 turn before the firing pin finds its recess and can move forward and hit the primer.
To me this is a very unlikely possibility... At best.
I never had any slam fire issues during my short service with the Garand nor have I ever witnessed one such incident in the 27 years after that, even when we organise big shooting competitions with the Alpini and use Garands.
Concerning the loading data, I can't say much. I have a .308 M1icon and load it relatively mild, but that is just me.
42 grs. of N140 with a 147 grs FMJBT bullet. Nato standard.