Thanks Keith, and glad to be here! 
I am not sure about whether the ammonium system is more or less difficult to clear than potassium-based or other formulae. So much depends on other variables such as paper, developer, and even, as Scott points out, on the water supply. I share your concern that the matter of clearing residual iron compounds is important to all variations of pt/pd processes.
I was lucky enough to work with Mike Ware and a number of conservation scientists at the Smithsonian and other national museums a couple of years ago, whose findings are about to be published in a large volume about platinum and palladium printing (I’ll be sure to post more details as soon as we have a publication date). Much of their work around residual iron in pt/pd prints may be summed up as follows.
The presence of iron compounds in paper fibers can lead to deterioration. The presence of iron compounds in prints can induce overall or localized print discoloration. Thus, for archival and aesthetic reasons, it is desirable to process pt/pd prints in such a way as to keep iron content to a minimum.
The team tested many formulae and combinations for clearing platinum, palladium and platinum-palladium (1:1 by sensitizer volume) with potassium and ammonium-based systems, development-up and print-out, on a number of different samples of the most suitable papers. It then ran all the prints through standardized simulated aging tests.
What we concluded was that the most efficient clearing sequence, regardless of sensitizer formula and development method, was the one of Disodium EDTA first, then hypo clearing agent / sodium metabisulfite, then Tetrasodium EDTA, using the strengths and times, with interspersed rinses, that I describe above. What is important is that this applies to whether or not one uses a developer prior to clearing, and that the sequence makes a big difference.
Another thing to note is that while many of the tests looked just fine after washing and drying, the aging tests began to display visual changes in clearing methods that did not use disodium EDTA first.
So, even if tetrasodium EDTA is somehow converted to disodium in the presence of citric acid (I don’t have enough expertise to comment on this, but will look into it!) it is not happening when disodium EDTA is most needed… at the first instance after development in development methods, or at the first instance of processing in print-out methods.
It does not take much to add the disodium EDTA to your work flow and is well worth the extra few minutes. Besides, it makes the final wash much more efficient and if water temps are kept close to process temperature, the final wash may be reduced to the minimum 20 minutes, thus saving water, and reducing paper swelling.
I hope this helps, and remain eager to know if Scott finds a solution to his problem!
I had similar problems to your about clearing Arches Platine, years ago. But this new batch, which became available last year, is wonderful. It clears without any problem, and to add to this, the paper is strong, renders astonishing dMax, coats well (I use a glass rod), and has, most important, a grain-free rendering of mid-tones and highlights. I am also managing to get a lovely range of tones going all the way to blue-black at the higher hydrations ( pt:pd at 1:1, ammonium system), but have heard from other printer friends that the potassium chemistry renders a very warm black only.
I’ll post an image in a few days of an Arches plating print.
