The solar controller will have two temperature sensors, one for the water and one that will go up by the solar panels. If you're gonna run the lines underground to the garage, I would run a wire to go to the roof temp sensor at that time. It will come with a control box, 3 way valve, and valve actuator. You're running into the same predicament that I've pondered with running everything before getting the controller. I would have to buy a 3 way valve to run the system, then get another valve with the controller package. I haven't found the controller system that didn't come with the valve, so I'll end up having an extra valve when I get the controller, and probably sell it on ebay.
You don't have to have any block valves on the solar system, but it wouldn't hurt to have a set on the lines to and from the panels in case you needed to work on them. Those could be added later pretty easily if/when you actually needed them. Unions would only be necessary if you were running the lines on the ground and wanted to remove them for the winter. They could also easily be added later if/when you needed them.
On your panel array drawing, the return of heated water needs to come off the top of the panels. The vacuum relief valve will be somewhere on the return line also. It allows air to be pulled in so the panels can drain out the return line back to the pool when solar isn't being used. The cold supply side will drain back through a hole in the check valve. You don't want it all rushing back at once, so you drill a small hole in the flapper in the check valve. As far as the check valve goes, a lot of the solar systems I've seen just have the ones like lowes sells. Here's a link to pooldv's build thread. You can see how his is laid out.
pooldv pool build
