[Alneyan]

I can only answer question 2, since there is no love lost between the strategical battles and myself. (And believe me, you do not want to emulate my own "strategies" when such battles occur. *Smirks*)

Sentry means that the ship will not move and will be considered "inactive" as long as no enemy enters in the system. Afterwards, the ship will carry out its orders, if it has any order besides sentry. The main use for this command would be to give orders for your ships when faced with an invasion, but it may be quite unreliable as you won't know *which* ships entered the system, how many and so on. (For example, you could order a Stellar Destroyer to blow up a star once an enemy enters the system, but it would occur if even a single scout enter the system, which isn't quite what you want)

Sentry can also be used to remove some ships from the "go to next ship" command, although fleets would work in this case as well. Lastly, all ships in a sector engage a battle with any enemy ship entering this sector, unless one of the sides has all its ships protected by cloaking. (Assuming the other side doesn't have a sensor able to see through the cloak) So you do not need the Sentry command to achieve this effect.