Both Webasto and Eberspacher need a good circulation to get rid of the hot water that they are generating., so you need to keep the flow going. If they don't have enough load, they will start and stop which will reduce the life dramatically.
1. Ideally the main flow should be in 22mm with 15mm T to each radiator.
2, Remove all thermostatic valves and leave all of the valves open
3, Make sure any branch through the calorifier is fitted with a 3/4 closed isolator to reduce the flow throught the coil.
4. Make sure the header tank is in the Return to the webasto, not the flow.