1: check the color of the flag in your quest inventory slot. just follow the order from top and the mine should be deactivated.
2: Moving fast and avouid being shot
No, just use the cover available, move in with one of your mercs, throw a few nades, move very close and finish them off with hip-shots. It's one of the toughest missions in the game and there are many tactics that might work. Try and try again until you find one that works for you.
3: Sometimes that happens, if you explain better which mission you're talking about I can prolly fix it in the code. Some of the random missions had the same problem and those have been fixed, I can easily do the same with this one if I knew which file to alter.