That's great that the deaths of fish have stopped. It still concerns me that they stay hidden. You wouldn't happen to see them in the middle of the night swimming in front of your powerheads, do you?
Hey, you stole my new website name! TytheReefGuy.com!
PE is always good to see on a new frag. I sometimes get maricultures that come in and don't extend their polyps for a month! Once they do, I stop worrying about them not making it.
Here are my thoughts on your reactors. Yes, take the biopellets and put them in their own reactor. It's hard enough to dial the biopellets in to have a nice tumble but to have to coordinate that with trying to fluidized GFO just sounds like a lot of trouble. Go back to the GAC and GFO dual reactor setup and get an independent one for the biopellets.
Another thought, I worry about you still running biopellets with an alk at 10 dKh. Normally, when you carbon dose, you want your alk on the low side, 7-8 dKh. You'll often get alk burnt tips otherwise... something to do with the amount of available carbon causing competing issues with the zooxanthallae and the actual growth of the coral. If you're not seeing any of these issues, then I guess continue what you're doing. I'd hate to have it swing you another problem by increased nitrates if the biopellets shut down. Just keep an eye on the acro tester piece and we'll see how much issues the high dKh combined with the carbon dosing causes you.
For carbon, I've been running it passively in a sock for years. I just throw it in a high flow area and replace monthly.
Have fun replumbing! I always end up with at least one slightly leaking connection and have to redo it.
Once you find out what size UV you are running, match it up with your tank size and we can see what the recommended flow is for it. AquaUV recommends you add it after any mechanical filtration so probably last in line before it goes back in your tank.