Hitting line speed with iperf and gigabit should be pretty easy, not hitting it means there's a LAN problem, and the actual speeds can then be slower than iperf, I suggest trying to fix that and it should help.
You can try mounting the filesystem read-only, and if it works then copy the data elsewhere:
https://forums.unraid.net/topic/46802-faq-for-unraid-v6/?do=findComment&comment=543490
That's not possible since parity2 is calculated in a different way, they are not interchangeble, but you can add parity2, remove parity1 and use only parity2.
You can try force enabling disk1 to try and rebuild disk3 again, but the diags shows the SATA link going up and down for multiple disks, you need to try and solve that first, are you using any power splitters? You can also try a different PSU if available.
Silicon Image, Inc. SiI 3132 Serial ATA Raid II Controller
This is a PCIe 1.0 controller, with two devices it will be limited to around 80MB/s, would recommend getting a PCIe 2.0 controller from the list on the 1st post, like an Asmedia 1062.
Nope, but there are a few NIC related threads in the hardware forum that should show up in a search, you can ask there for a recommended model, mention what you are looking for, e.g. gigabit or 10GbE, UTP or fiber.