Vacuum release valves can be put almost anywhere in the solar plumbing. It is simply to break the vacuum so the panels will drain away from the valve. However, the closer you put the VRV to the pump, the lower you can set on the pump RPM and still keep the VRV closed. Some installers will place them a few feet above the pad so the RPM can be set really low. You might find that energy efficiency trumps heat transfer efficiency. Plus the panels will probably produce more heat than you need.

As for the 3-way valve, normally a solar valve is used for this. It has a built in check valve which allows the supply side to drain when shut off. You can also use a standard 3-way with a hole. It accomplishes the same thing which is basically a path for drainage. Another way to do this is to have supply/return bypass pipe with a valve so there is a path for drainage when the solar valve is off.

As for your last point, yes the check valve will be held closed when solar is shut off so normally the panels will remain filled to a height indicated by the filter pressure. If the water temperature is at 140 F, then this could be a problem for the PVC pipe as it will soften some at that temperature. If the VRV is plumbed too low, when the pump does shut off there can be a high vacuum at the hot PVC pipe and the pipe could deform. So in this case, you are better off plumbing the VRV close to the input to the panels so the vacuum cannot get too high.

However, another way to manage this is to step down the pump RPM when solar is disengaged so that the panels will drain at least part way down so there is no water in the panels or pipes on the roof.