Legally, you can go to military duty at any time, but during training I (and everyone else) usually reschedules it unless it happens to fit into a good place in the training program.

It's hard for the airline to do "makeup" training...they are obligated by FAA rules to ensure you attend a certain number of training hours, and there's usually little to no padding in the schedule. If you skipped a week of ground training, you'd probably just have to wait for the next class.

I would not want to get interupted in the middle of sim, very bad idea. It would mess up your progress and leave your sim buddy hanging.

IOE is no big deal to reschedule, as long as you get it done within 120 days of the sim checkride it's legal. You also need 100 hours within 120 days (consolidation), so I wouldn't go on extended orders until after you get 100.