Possible spoilers for Carrion Crown adventure path await:
There's a particular construct in the Trial of the Beast that is both 'Slowed as per the slow spell' when affected by cold or fire damage, and 'can take an extra move action during its turn each round' as a supernatural ability.
My question regards action math:
Logically, a creature capable of making a move action and a standard action should be capable of combining those into a full round action, such as a full attack.
Rules as written, the the slow effect's application of the staggered condition prohibits the creature from taking a full round action and the entitlement to an extra move action does nothing to alleviate this.
How should this be run?
Doesn't work, per above. This isn't a case where C = (A + B). It's a case where you can do either (A and B) or C. They're alternatives, not equivalents.
In a normal round, you can perform a standard action and a move action, or you can perform a full-round action. You can also perform one swift action and one or more free actions. You can always take a move action in place of a standard action.