[Alneyan]

Nebulae creators are cheaper than regular Star Destroyers, not more expensive; if all you need is to deal damage (and you have the research), they are the best option. If you are planning on rebuilding in the destroyed system, the regular Star Destroyer might be better (though removing the nebulae isn't very hard when you have Stellar Manipulation vessels galore).

2) So long as you start on the star (I think), you can move around until the end of the turn, and blow the star by coming back to the star sector.