Thanks for the picture and the additional info. It may be best if you can post a picture of your overall pool, ensure it shows this basin and also the larger water feature.
With regard to the return pipes. That valve that is partially open which controls flow between the spa and the pool is set in an overflow position. Basically, it is feeding some water to the spa such that it flows over into the pool. This keeps your spa water sanitized with the rest of your pool. However, it does not need to be on constantly unless you wish to have the spa overflowing into the pool. You may have an automation system (either an app on your phone or a control panel in your home) that can be set to schedule this overflow feature. You only need to move water through your spa an hour or 2 a day to keep it fresh. If you have automation, please take a picture of that panel. It would on a wall next to your equipment.
The overflow feature is why you have return flow to the 6 jets in the spa. They only have minimal pressure because the valve is only partially open. Also, the pressure & flow to the spa jets declined when you opened up that valve to the far right that fed flow to the basin. That was expected because your flow is now to 3 different pipes - the spa, the pool and to the basin. Not sure what the basin is there for other than a mini-water feature into your pool. The photo of the overall pool would help.
The 6 returns to your pool with only 4 putting good pressure and 2 are not. Just to confirm - do all 6 have some sort FLOW but it is higher in 4 and much less in 2? Or are the 2 with ZERO flow? This could indicate a couple of different scenarios. I think one of those none working returns is your suction for the cleaner. Or can you identify that the cleaner is hooked up to a different port? The other non-functioning return could be an overflow feature that keeps your pool water at a set height. But this return would be near your water level and not 18 inches below the water level. Again, some photos of these 2 non-flowing returns and an overview of the pool may help. On your photo, please identify which 2 returns are not working and the position of the 4 returns that are working. A picture of a sketch of your pool with those notations would suffice. The other scenario would be that you have a blocked return - but hopefully that is not the issue because most return lines on built on a loop - so the 4 or 6 returns act as one pipe and the only way to isolate one is to put a cap on the return in the pool wall fitting. So, to rule that out, verify that all 6 returns are open or have no cap in them. Also confirm if they are all at the same relative depth below the water level.
Sorry to create so many action items but need that info to try to narrow down the conditions that could be causing this.