1. Is the AXSR stock firing pin (Large or small or whatever it is) ok to use with .338 LM, .308, and 6.5CM? I plan to shoot primarily manbun and .308, end the day with some .338 probably. Don't want to pierce primers and it seems like people have been having that issue with 6.5 specifically? Or was that just with the older MC?
Just the MC. All AXSRs are small firing pin-only, so they don't have any issues with primers on any cartridge. MCs came with large firing pins, but they eventually came out with small firing pins and matching small pin bolt bodies to fix the primer piercing problem in certain cartridges. It should be noted the MC was originally designed for the military, and only to shoot .308, .300WM, and 338LM. There were no pierced primer problems with any of these cartridges. It wasn't until they started offering barrels in other chamberings (much later) that the problem arose with the large pin.
To the first part of your question, yes the stock SR firing pin assembly (again, they're ALL small firing pin) will fit ALL of the 3 SR bolt body sizes, and don't have issues with primers in any chambering. You only need the stock firing pin assembly that comes with the rifle, and then the other 2 sizes of bolt bodies that didn't come with the rifle, and you're set up to shoot the whole array of calibers (you will also need the other two sizes of mags as well, and obviously whatever barrels).
2. Is the AXSR too big to fire out of various firing positions like kneeling, siting, standing? It looks really big and awkward in some pics and then manageable in others. I plan to use this for all shooting that I will do, including plinking, hunting (18.5 proof barrel), and potentially competition like PRS which I am learning about.
It was designed to be a military sniper rifle. The closer your planned use is to that style of shooting, the more optimal it becomes. LR/ELR from bench and prone is where it is unbeatable.
3. is the AXSR being replaced at shot show and will it be impossible to find conversion parts as a result?
Don't know when the next design will come out, but probably not for a while if I had to guess. But it won't matter, you can still get parts for the MC and the SR has been out for a while.