I suspect from your photographs that your granddad was in the regimental cannon company (that's likely why he has photos of a gun crew firing a 75mm cannon). After basic training which was not usually done with the division he went to speciality training and then to the division or, to the division and then to speciality training. In the case of artillery (and many other specialist trades) it did not always happen at the same place as the division was officially located.....You were sent on temporary orders to a school to learn the skill and then returned to the division.

The CIB is earned for being in combat. I suspect in this case he was seconded (that is pulled from what was supposed to be his job) to be an infantryman in the regiment. This is quite commonly done. Since you know his MOS was 835 Supply Clerk this is very, very likely to happen. Clerks are seen as nothing but an immediate pool of replacements in a division in combat, particularly ones that are not in some critical position.

As for the lack of the other campaigns on his record, this is very common. Clerks making records entries often left stuff off, got it wrong, never made the entry, etc. I had to have my own DD 214 retyped five (5!) times before I'd sign it as correct. Many soldiers really didn't care at the time about their records being straight; just surviving was far more important.