So I've been having trouble finding information on stacking enhancements. Can you stack something like "orbital bombardment" and therefore your units gain 2 black star bursts? What about "heavy blaster emplacement"? With "proximity mine" can you have two on the table and sacrifice them both to do 4 damage to each attacking enemy unit?

If it says the unit gains a blast, like orbital bombardment they will stack.

The card text for "Obrbital Bombardment" reads:

Each unit you control gains [blast Damage].

So for each Orbital Bombardment you control in play each unit you control gains a Blast. If you have 2 in play they will gain one blast for each (totaling two).

This is similar to the effect from objective "Across the Anoat Sector". If they change the value to a hard number, such as "Raise the Stakes" or "The Moorsh Moraine" it is raised to that level (but does not stack).

The card text for "The Moorsh Moraine" reads:

Your unopposed bonus is raised to 2.

Becuase your unopposed bonus is raised to a static number it does not stack (also this enhancement is unique), but if it was stolen by the LS it would not stack with Raise the Stakes.