I'm an enlisted guy. The following info is baseline, general stuff, doesn't apply in ALL situations.
Everyone does a desk job at some point. Officers will be in staff positions much sooner and for longer periods during their careers than enlisted guys. Even as infantry, you'd be a PL, then an XO, then Company Commander so on and so forth. The only time when you would be on the ground is while you are doing your PL time. Everything past that is staff, command, desk job stuff. Later on once you make Captain and you want to try out for SF, you'd do all your training and then go to Group, where you would get about 2 years team time, then begin your Company Command as a Major. Generally if you want to be on the ground, patrolling, conducting combat operations you could get about 5 years worth of that stuff as an officer, as opposed to roughly 15 years as an enlisted man. Again, this is just a rough estimate from what I have gathered in my short time in the military.
1) You can reclass. I don't know the ins and outs of this.
2) Most SF officers are prior infantry. It doesn't matter what your MOS is though, anyone can try out and non-infantry officers make in through the course without any issues.