It does not appear that I am meant to keep altums.
The importer suggested 25% water changes twice per week and I am doing so, yet after nearly each change I find a carcass. The new water is aged and pH matched to the tank and everyone looks healthy enough at all times. Short story is I now have a total of four altums left. If I can keep these four I will have a total of six angelfish (includes two scalare that spawn regularly) which won't be as many as I envisioned, but will still make a nice display, especially when fully grown.
I feel somewhat limited by not diving into an RO setup - a difficult arrangement in an office setting - but I accepted that restriction and gave them my best effort and this is what I have as an answer. Sometimes our reach exceeds our grasp.
Plants are stable. I detached and gave away the wisteria on the left while the patch on the right now covers 50% of the surface. If it takes 3 months for a CO2 scape to show its stuff, I expect at least 6-9 months before this one is where I want it. Let's hope altums are still in the picture.