Is there a way to select 2 or 3 agents, or can I only select 1 or the whole group?

Also when I try to play some of the missions (Pacific Rim and Siberia for instance) it loads a featureless field of grass and then automatically says mission completed. Does this happen for everyone?

1) I think you can only select each agent individually, or the whole group.
2) This is not normal, I have never had this happen. Maybe try reinstalling the game? (I'm on a Mac, and the save games are saved in a separate folder so they wouldn't get lost, I assume it's similar for Windows but you could make a copy of your saves and rename it just in case if you reinstall).

1) Try to select all squad and then deselect one or two agent, I don't sure if it will work.
2) I beat the game many times and that is never happen to me