I know how you feel. I also have a 12 long with a Sat+ and did a dry start to grow a carpet of HC. Mine yellowed, thinned and died out in the months after submerging.
I was waiting for it to bounce back, since my other plants were doing very well. Unfortunately, after three months, there was no sign of a recovery and far too much rotting plant matter at the bottom of the carpet to ignore. I decided to pull it all up and start over. Hopefully yours doesn't get to that point.
My new HC is doing much better after making some adjustments. Here's what I've changed:
I put a powerhead (Hydor Evo-Mag 180) at the far end of the tank to push water toward the filter intake. That half of the tank was pretty much stagnant. I think the low circulation was the main thing that killed my carpet.
I increased the CO2. I've been slowly dialing it up beyond lime green on the drop checker. I'm trying to find out what is the highest concentration the fish can tolerate.
I'm also cleaning the tank as frequently as I can now to remove dead plants and algae. Cleaning the filter and powerhead helps keep circulation up and checking to make sure the reactor doesn't need to be bled keeps the CO2 consistent.
I upped the light to 10 hours, and put the Sat+ on its brightest setting. I think the sudden drop from 14 hours/day while dry starting to 6 hours after flooding was a bit of a shock for the the HC. Even with a 10 hour photoperiod, I have no BGA or hair algae, whereas before I had quite a bit. I'm attributing the lack of BGA and hair algae to the increased circulation.
Anyways, I hope some of this might be useful. You have a great looking tank, best of luck getting the carpet to thrive.