I just read your whole story and holy cow you went through a lot! Amazing job sticking through. Sorry to hear that coral never made it. I have a 170 as well and a 10g coral tank. I went through simar issues with the 170, and now am in a battle with gha. I'm starting to turn to the skimmer... I never hear much problems with yours. How do you have your skimmer dialed? I've had the foam towards the top of the come, it blows over. I've had it starting at the base of the cone, it never makes the cup. Now ive been in the middle of the cone and it makes an ugly goo mark, but not as much as I'd expect and very little in the cup.
In the year I've had it, I've never seen a spec of corraline, so I wonder if I'm facing a bacteria problem too. My readings are always zero. I've just added phosguard to the picture and see the gha starting to die back. I do 25% once a week and remove manually as much as I can. I would love to try coral in the tank some day, but not till the gha is gone...
I plan to take some pointers from you and your journey, thank you for your insight!