I surmise the problem may be the reverse of that described. While it is true that the ink is clumping up, that may be due to self-assembly rather than the solvent being too active. Because you are describing unknown (to me) polymeric materials, could the solvent interactions of the ink be weak compared to the ink-ink interactions? That is, while the solvent intercedes with the carrier polymer, it does not interact strongly enough with the ink. Simply decreasing the number and availability of solvent electrons or protons may slow the solvation of ink-carrier polymer complex but doing that appears to simply not work. I presume other solvents were tried. I am guessing this solvent mixture actually does something, but simply does not lead to a successful solution.
I might try to find solvents that can dissolve the ink. For that, you don't need the polymer or the ink-polymer complex. I would start with high concentrations of that solvent and begin to increase the amount of the cellusolve solvents to break the complex.