Couple of things....
You said "noticeable discoloration" during selenium toning. What kind of discoloration? Any stain like mark usually means insufficient fixing and washing. That maybe where your problems are starting.
I don't know Adox but some paper do not respond to toning all that well. Ilford MGIV is one of them. But, it won't stain like you are saying. The silver part gets slight tinge of brown/purple kind of color. White stays white.
Because you kept calling it stain, I'm guessing white part is discolored. That is likely caused by insufficient fix/wash in the first stage.
Sodium Sulfite stop bath needs to be MUCH stronger, like 10% for it to work. (I just learned this myself) So take 100grams of Sodium Sulfite and dissolve it into 1000cc of water. What you have is more less HCA strength. In that dilution, it will have the opposite effect. Strange enough, when polysulfide toner is diluted to certain degree, it speeds up. I used to see my toning do very little and as soon as I soak it in water bath, it goes BOOM!