nitrates has been stable at about 2 ppm
Remember that's 2 ppm +/- you kit's accuracy rate, which might be > 2 ppm.
In other words, 2ppm can actually be zero. Check out your test kit manual to see if it tells you this +/-....it should.
Until you confirm one way or the other I'd suggest assuming that it's zero – performance of PO4 backs this up, as does the condition of your corals. (And remember this isn't the only factor at work.)
They seem to be in a reasonable place
I'll take your word for it. In that case, stability of light is the #1 concern....roll back any/all changes and throw away the password to your lighting system. :)
Re. Flow, I estimate about 50x flow, which I think is sufficient.
"X" numbers like "50X" are good for almost nothing.
At most a number like that is merely a starting point to get you the correct number of pumps during the planning stage. Reality trumps the "X" number in 100% of all cases.
A big part of the problem with the "X" number is that it's based on another almost-useless number: GPH.
The most useful aspect of flow is not quantity, or GPH. It's velocity, or centimeters per second.
Flow is invisible, so it's very hard to know what's going on. Especially for a beginner.
Here's my analysis:
The main clue is two little mp10's in a large (36"?) tank.
They're good for everything that's within 12" of the pump. Everything outside this area of effect is in marginal to very marginal flow. Part of the reason for this is that these pumps are extremely low velocity.
I don't see them in the vid – where do you have them located? Willing to switch or add to what you have?
Here's a video on flow that explains a lot and has a great visual demonstration of flow as well: