It shows the lift pump is working basically.Or in itself is not causing what you are experiencing. Close off that return line to eliminate a totally failed pressure relief valve on the injection pump line causing issues as a test. No harm will be done on a factory lift pump . On an electric pump I am not as sure about damage to the electric pump.

Car may still have contaminated residual wvo components in the fuel system that could have hardened up with time. I doubt that the solvent flush you have been using would address animal fat residue properly if present. The lift pump on an ex veg oil burner may need cleaned out properly and the piston and bore inspected for damage as well or being partially gummed up.

At some point I would give that injection pump a good 24 hour solvent soak in place. Cars that have been on wvo may need a really good fuel system cleaning. The residual stuff turns into a form of glue with time.