Thanks for the pics. I'll defer to those here that know how your multiport valve works. But you've only got one.
You find return manifolds on pools where the "output" of the equipment on the pad splits off to various places, like maybe the pool and a spa, and perhaps some water features (fountains and bubblers, etc). Such a manifold will have a few valves, one or more for each of those functions.
You don't have a true return manifold, because it looks like you only have one pipe going back to your pool, which would be for the returns (some people call those "jets"). You have just the one orange-handled ball valve on that return line. "Manifold" implies more than one valve.
I'm guessing, but it looks like the other orange-handled ball valve leads to a pipe that is used when you have your multiport valve in its "to waste" position. Typically, you wouldn't need either ball valve, unless some or all of your pad's equipment is below the pool's water level. Is that the case?
No matter, if the arrow in the following pic is pointing to the pipe that leads to your pool's returns, then the arrow is also pointing to the ideal location for your SWG, except that there is no room for it. I don't think anyone here is going to endorse locating the SWG where you want to put it, because that would send highly-chlorinated water into your multi-port valve, which is not good for it.
