We inherited the spa when we purchased the home last year, so I will try to answer the questions as best possible.
1) Filters were cleaned. Tomorrow I will try removing them to see if it impacts the flow.
2) Not sure. There are several pumps due to it being a swim spa. How would I be able to tell.
3) Yes, I have a multimeter.
4) I don't have a wiring diagram, but have posted some photos of the controller board, heating element, and display.