Still a bit confused on some parts of his kit…

Need some help with understanding certain parts of DP’s kit:

1. Can you infinitely goomba stomp people? I’ve had a DP goomba stomp my backline 5-6 times and killed everyone this way.

2. How does his dash refresh while using dual katanas work? I’ve see clips of people doing multiple dashes back to back, but at most I could only do it once or twice. Does it have something to do with the chevrons you get after dashing?

3. What does his ult actually do? In all three roles.