Thanks for the feedback. Please check my understating as I have a different understanding of how the 3-way valves work:
1) This is a spill over spa, but when the pump is off the right most valve should also be off. I will be using Jandy 3-way valves and my understanding is they have an off position where water will not flow thru either outlet. There could be a failure situation where the system thinks it is on, but the pump is off (say it loses power) and in this case I could see where the spa might empty.
2) The 3-way valve can allow water to both outlets simultaneously and also adjust/restrict flow to an outlet. So the right most valve in pool only mode would be set to have say 90% flow to the outlet for the pool and 10% to the outlet for the spa.
3) In terms of home running the returns, I completely agree. My only concern is I've also heard it's best to use a loop around the pool for the returns so you achieve equal flow out all the returns. I believe these 2 things contradict themselves. I've also read you can change eyeball sizes to adjust the flow at each return. I'm thinking I home run the returns and then adjust the flow based on eyeball sizes...can someone confirm this is possible?
4) I threw in the vacuum port for a cleaner. I will likely buy a robot, so I may remove it.
Sorry for the delay responding,
1, Power outages were the first issue that came to mind, you would not want the plaster in your spa to dry out during some random summer power outage while you were away from home.
2, That is fine with a manually adjusted valve, but valve actuators move until they hit a stop, which can be a pre-set point, but then things can get complicated taking the actuators apart and moving stops, when say you may want to change that from 90/10 to 80/20 or 100/0 for whatever reason, a manifold style distribution may work out better here. Here is a link to the process of adjusting actuator cam stops
How to Set the Cams on an Intermatic Pool Valve Actuator - INYOPools.com
3, If you home run the return lines you can adjust flow on the lines by partly closing the manual valves (on this topic if you don't want to spring for the good Jandy Neverlubes, Hayward and perhaps others offer serviceable ball valves which are made to a higher quality than the cheap red handled hardware store valves. Hayward makes 3 or 4 different models (threaded, glue in, etc.), some have unions built in, one thing I like about them is they have removable handles, so if you have something you want to tweak a certain way and don't want anyone messing with it you can just pull the handle off. You can also adjust flow by changing out eyeballs, either inline valve or eyeball, both work by restricting flow.
4, It can be handy for traditional manual pool vacs, but plugging into a skimmer socket is not that hard to do either, also with liability what it is these days and the entrapment hazard (even if remote) I would not install one.