Need 3/4" plug for Rainsoft Q2 water softener

I went back to the original cross threaded plug and worked on it to "repair" the threads. I got it sufficiently repaired and was able to screw it back in adequately.

I have a few other questions about the system. I inherited this system when I bought the house. It consists of two cylinders. The left cylinder has the controller mounted on top. The right cylinder has the timer mounted on top. There is an air gap valve behind the left cylinder that is slightly cracked. I suspect that I either need to replace the air gap valve or repair it (seal the outside with a good plastic glue. I assume that if it is sucking air through the crack in the air gap valve it will not be pulling the brine like it needs to.

I'm trying to understand exactly how the water softener is configured. Is the left cylinder (with the controller) the resin cylinder? Is the right cylinder just a storage cylinder of softened water?

At this point I have the water softener bypassed. Is there a good article on troubleshooting a water softener? What is the best way to determine if the brine solution is flowing into the resin tank?

I have 13 GPG water hardness. It would be beneficial to get the water softener working correctly.

Thanks in advance.