I'm pretty sure that's the bonding wire. I believe it's just supposed to electrically ground the pool, so it just needs to be in the ground. I think it actually runs through the ground all the way to your equipment pad, where it also grounds the pump, lights, and anything else over there. So that all the pool and equipment are grounded to the exact same potential.
So, in my pool, there is no suction directly to the maindrain. The maindrain is hooked up to the skimmer, and the skimmer is routed to the pump. So apparently they still hook up the skimmer to the maindrain even with suction, to allow water to flow up into the skimmer from the maindrain if the water level goes below the skimmer. The reason why it's "collecting dirt" is because there's not really any suction or pressure there. It's just open to allow waterflow. If you have suction in your maindrain, you can probably blow it out with a hose every a couple times a year to push the dirt into the maindrain.
have two skimmers, one has two holes and the other only one. Why are there two holes at the bottom of a pool skimmer was one of the earliest questions I had when I first got my inground pool and started to clean it....
easypoolcleaning.com
For the last one, my acid wash crew took care of stuff like that. (And griped about the plaster crew not taking care of it, but the plaster crew didn't have a pressure washer, so I'm not sure what they were expecting.) Then again, your crew also didn't patch that line, so it seems like they were slacking a bit on your project. You can still pressure wash it now, it's just that all the stuff will go into the pool.