Can a 0 might reflection unit (copied something that got star crossed or gust or killed) can conqueer or hold ?

Yes. A 0 Might Reflection is still a unit, and Might has nothing to do with Control, Conquering, or Holding — same as Zed's 0 Might Shadow Clones (FAQ #12473). It can walk onto an empty battlefield you don't control, win the Non-Combat Showdown by being the only unit present, establish Control, and Conquer for a point (Rules 348.2.a, 469.1).

Two important caveats:

• If the Reflection already finished copying before the original got Star-Crossed/Gusted/killed, it isn't 0 Might at all — a finished copy effect persists, so it keeps the copied unit's Might and text even after the original leaves. The "0 Might Reflection" only exists when the copy was stopped in response (copy target gone before the effect resolved, FAQ #10681).
• A Reflection from Mirror Image/Deceiver always ends up with Temporary — the grant isn't tied to the copy succeeding (FAQ #11492). Temporary kills it at the start of its controller's Beginning Phase, before scoring (Rule 816.1.b), so it will die before the Hold scoring step. If it's alone on a battlefield when that happens, you lose Control in the cleanup and get no Hold point — the Reflection can conquer during the window it lives, but it can't be the unit that Holds. (Without Temporary, a 0 Might Reflection would hold fine.)

Combat is where 0 Might hurts: if the battlefield it moves to has enemy units, it adds 0 damage and dies to anything assigned to it — conquering only works against empty/uncontested battlefields.