By reducing density of the negative, the solvent will reduce the density of the positive, since it affects non reduced Ag+ : if I understood well, with potassium thiocyanate, SCN- reacts with the silver halides complex Ag(X) and forms Ag(SCN) + X-, thus making less silver prone to reduction available, which means less density, whether it's on the negative or positive. And since the amount added to the developer is around 2-5g, it's easy to put too much of it and spoil the density. That's what happened to me...but I guess if you add just the right amount it should clear highlights without spoiling the darks, increasing contrast. There's also the sodium thiosulphate which reacts the same, but one is stronger than the other, and I don't remember whether it's KSCN or Na2S2O3.
Another variant I read about is using the solvent as a separate bath after bleach and clearing, what would this change? Maybe more control, since you can work in daylight at this stage?
As for the film in bulk, I buy it from Wittner Cinetec, Germany. They have Orwo UN54, Fomapan and Agfa Scala 200X.