wow - such in-depth discussion - this was great!
Here's the update (and my thoughts based on what I've read)
I did a 90% water change and added an external canister filter (carbon, filter floss and sponge media). The water is mostly clear and stable over the last few days (not getting cloudier again). Now that I could see into the tank again, I could trim out some of the dying leaves.
My guess is that the good bacteria never got the edge when the tank cycled. The internal filter sponges just didn't do the job. Once the cloudy bacteria started, it caused a cycle of plant dying, which just fueled the cycle more. Add a little green algae bloom along the way, and the whole system was out of balance. Even though I changed 20% water, ran more lights, ran UV sterilizer, it was never enough.
I'm going to leave the external canister running, plus the addition of the Seachem, to try to keep the edge for the good bacteria. All the plants are pearling again, so that's a good growth sign.
Thanks again !