Do mastery effects and effects from other sources stack, or do you take the highest?  For example, a Barbarian attacks with a Push mastery weapon, and also uses the Forceful Blow Brutal Strike.  Does the target get moved 25 feet (10 for Push + 15 for Forceful Blow), or just 15 (the highest of the two).

I know in general effects with the same name/source don't stack (so getting hit with 3 crossbow bolts and the Slow property in a round is just going to slow you 10 feet, not 30), but here you have the same basic effect from two different sources.

You'd have to look closely at the wording for how and when each effect is triggered.  As long as they both can be applied then they would stack.  My hunch is that these effects would stack.

I'm not aware of any new rules that would prevent them from stacking. Strictly speaking, they're still resolved one at a time, and the player would determine the order in which Push and Forceful Blow are applied per the Simultaneous Effects rule in the glossary, but applying Push after Forceful Blow would be quite silly.