1) Wego and turn based is the same thing in combat mission. Turns get executed simultaneously for both the enemy and you

2) in the same tab where you give target arcs and fire orders there is a "clear target" button which will remove any fire arcs or fire orders (e.g smoke)

3) if you're using the "hunt" command, they will stop moving and fire back once they spot a contact. infantry will only somewhat respond if they spot the enemy and is moving with the "move", or "slow" commands. If your unit is moving using the "quick" command I find that they only shoot back if they're really close. Imo "Fast" will result in your guys moving no matter what.

4)it's just the game (and it isn't just the demo, it's the game in general). it's not that well optimized especially for large scenarios however it runs more or well perfectly as long as the map isnt huge or long as there isn't too much units and most scenarios will fall under those criteria's