Well, I am a newbie here,but I'll take an admittedly ill-educated stab at answering your questions.
One of the demos for CMSF shows US troops firing three round bursts with their M4's, so I would say yeah, you will get to see different rates of fire. Taking a stab in the dark here, but I'd say that the TacAI will decide what fire mode to use - leaving it up to the player would probably be too micromanagerial (is that even a real word? ).
Taking another stab in the dark, I doubt weapon jams will come into it. Again, too into the nitty gritty for a game that's aiming to represent company-sized actions.
Re troops firing on full auto - have to say that I've never seen US troops fire in bursts from M4's/M16's in footage from Iraq/Afghanistan (which might just mean I'm not observant enough). Seen a lot of rapid firing on semi, but no bursts. From my own (limited) experience with the British army, full auto fire is tightly regulated, and only really approved of in certain circumstances, namely anti-ambush drill, short-range suppresive fire, and CQB (even then short bursts). It's just a waste of ammo otherwise, and best left to weapons like the SAW and GPMG/M240 which are, after all, designed with the purpose of blatting away on full-auto.
Our battle drills tended to concentrate on rapid fire (roughly one round every two seconds) or deliberate fire (one round every six seconds). You're also taught to count the number of rounds fired as best you can - not easy to do when on full auto.
My 2 pence, though I'm sure the more experienced (and indeed still serving) members can confirm/support the above (or totally blow it out of the water if necessary!)