Staining comes from incomplete washing before toning. Do not harden the prints or you can unharden then in a bath of sodium carbonate. Do not add acid to rapid fix. Use without it, film or prints. Hardener makes washing, toning, and spotting difficult.
Selenium can tone straight from the fix, NO rinse, no anything . Mix selenium with HCA. Then do archival wash.
To tone a print later, it must get a thorough and complete wash to remove all fixer.
My Ektonal is in a can and is 40 years old. No a and b parts. DuPont Defender developers like 53d, 55d, came in a can that appeared normal, but when you opened it there were two sections. I must dating myself pretty good here. Ok, I`m 63 and have been doing this since I was 16.