Hi, Phill. I see you're busy printing away!
A yellow stain is always going to be ferric oxalate trapped in the paper. I'm not sure how this would effect the kallitype process, but I might suggest adding a small amount of EDTA to the ferric oxalate solution. About .5 g per 25ml would be more than enough. I would suggest trying it with a very small amount of ferric oxalate in case the EDTA precipitates the silver nitrate from solution.
We find that many papers are difficult to clear completely, so adding a small amount of the EDTA to the sensitizer means there is already clearing agent in the image. The EDTA is also a chelating agent and aids in the solubility of iron compounds, so it's used in many fertilizers to help the plants absorb iron from the soil.
I've only had experience using Borax and Sodium Acetate as developers for Kallitype, but I'm suprised to read that Sodium Citrate is causing the silver to precipitate.
It's a silly question, but have you tried developing your images upside down? Many gum printers I've talked to will do that to limit staining. Perhaps it could work as a fix for you.
I'm always a little skeptical of using acids as clearing agents. I've seen Daivd Michael Kennedy pulling his hair out using HCl as a clearing agent. Acids are used as hardeners in silver fixers. The literally harden the gelatin on the surface of the print. Starches, gelatins and gum arabic are all used as sizing agents in modern papers. These compounds can harden when exposed to acids and trap you emulsion in the paper. This leads the printer to harsher methods of clearing.
I recommend to my 'hard luck' cases a combination of potassium metabisulfite and sodium sulfite to clear the most stubborn prints. 20-50g of each per liter of water will clear a tattoo off the back of your hand! Not really, but almost!
A funny anecdote, and then I'll leave you alone: My wife bought a very nice, and extremely expensive set of sheets for our bed. About six months after she bought them disaster struck.
One afternnoon I hear a scream and whimper from the laundry room. I run in expecting to find her bleeding on the floor. Instead, she's holding her sheets in one hand, and a red sock in the other. She had left the sock in the washer from a previous load and had missed it when loading the whites. There were streaks of red on her sheets and she was nearly in tears.
"I'll be right back, I'm going to work, you find grab the mop buck from the garage and wash it out!" I yelled, as I grabbed my car keys and ran out the door.
I returned home with 250 g of pottasium metabisulfite, 250 g of EDTA and 250g of sodium sulfite. I filled the bucket with about 6 liters of hot water from the tap. I stirred in the contents of each container I procured from work. The sheets then soaked for about 2 hours, after which I dumped the whole contents of the bucket into the washing machine. I set it on HOT and punched start. "Just you watch, honey, you can clear the worst pigments out of a gum print with the stuff I brought home!"
"Huh?" Was her reply.
45 minutes later I was a hero. My wife lifted the sheets from the washer and gasped in amazement.
"Wow! Not only did it completely remove that stain, but I've never seen these sheets come out SO WHITE!!" She said, with an incredible sigh of relief.
I decided at that point that someday you'd see me hocking Sullivan's Super Miracle Laundry Cure on late night TV. Maybe when advertising rates get cheap enough?
Thanks for reading.