"We have a crowdsourced research project for our ISEE-3 Reboot fans. One of our volunteers, Karl-Max Wagner from Germany has an interesting idea. Did the Nitrogen pressurizing gas dissolve in the Hydrazine in the tanks?"
Update: We spent all day yesterday with space propulsion experts. We have identified a series of options including hydrazine tank heating and a long series of pulse attempts to (possibly) clear the lines. We have most certainly not given up on this spacecraft yet. It is doing science and will continue to do so for years to come.