Do Stat-buffing Moves Still Only Work in PvP?

Yes but a defender rarely uses a move twice.

They can, yes. I don't remember how many times they can stack before they hit the cap, but it is entirely possible to get a stat buff to proc multiple times. At a 10% proc rate, it's rare that the buff will hit multiple times, but if it does, it makes a massive difference. I believe each application is a 50% increase to attack/defense, so its pretty significant.

I fought against a Giratina recently in ultra league that got the buff to proc twice on the first two Ancient Powers it used. Needless to say, I did not win that battle lol