I added some unions around my filter ... not really needed, although since the threaded insert to the multi-valve has a drip. I can open the union, tighten the fitting and then reconnect the union. They are really more useful for people who break down the equipment for winter storage or if you have a multi-valve that is on top of the filter that has to be removed to get inside.
The check valve after the heater would prevent the flow from trying to enter the heater backwards when the 3-way to the heater was turned off. BTW, I do not think your automation would actuate a valve to open the heater when it was going to be on. Really I am not too sure the best way to plumb this one. Many options would work.
My heater originally did not have any valve and all the flow went through it all the time. I was advised I could reduce the headloss by putting in a heater bypass for when I did not need the heater. When I re-did the pad I did not put the heater back in. But, I was planning to do just like your drawing. With 2 ball valves near the heater and a Jandy 2-way between the heater pipes like our drawing ... still may do this. You have to decide if you want to be able to stop all flow through the heater of just always leave some going through it.
The P4 will control 3 valves. Valve 1 and 2 are normally slaved together (suction & return) and would move when you hit the pool/spa button. There is no Valve 3 button on the control pad to manually press. As I understand it, there is an option to make Valve 1 and 2 move independently and controlled by the Aux 1 and Aux 2 buttons ... I think this requires the use of 2 of your 4 high voltage relays as well. You could "fake" it a little by making the controller think you have a pool/spa combo, but only hook the first 3-way up to Valve 1 and leave Valve 2 open. Then you could have it automatically turn this valve at some time or just push the pool/spa button to move that 1 valve. The 2nd valve would have to be hooked to Valve 3 (since you do not want it slaved to Valve 1). In the programming you can set times to activate this valve, but looking at your buttons, there is no valve 3 button, so you can not manual decide when to turn valve 3.
I am not getting a remote, so I figure, if I am going to have to walk to my equipment to push the buttons to change things, what is the point of actuating the valves ... I am already there, so I will just turn them myself. All I am looking to do is actuate a solar valve, so I could make that Valve 3, but then I can not easily manual over ride by pushing a button. So, I could make Valves 1 and 2 work off of Aux 1 and 2 relays and I could push the button to override ... BUT then I loose 2 of my 4 relays. Since I have a 2-speed pump, that would use the other 2 relays, leaving my none left for the pool light (don't care since the switch is by the house anyway) or any landscaping lights.
FYI, The PS4 controller which is an extra $250, adds control of a 4th valve AND there are more buttons which include overrides for all of the valve controllers.
Given my limited desires for actuation (really just want the SWG and solar controllers) I think I can get by with the P4, and might be able to control a water fall valve by making it my Spa. But, like I said, I am not spending another $500 just to have a remote control ... for money I will just walk over to the equipment to push buttons or move valves.
You just have to really think about what you want to accomplish with the automation.