What is Residual Hydroquinone, never heard of it ? If you stick the term into Google it refers to thev Photoformulary and skin lightening

It should mean the very slight residues of hydroquione left in an emulsion/paper base after processing.

Ammonium Thiosulphate (cryst) is readily available in Europe it's not so much unstable as hydroscopic so attracts water and tends to end up as a sticky mess, so for commercial use it's usually sold as a solution.