The pumps can get gummed up after being inactive for some time, there are two ways that may free it, a few choice taps with a hammer or application of heat with a few taps. Usually it is simpler to just replace the pump though.
It is possible that the fuel lines were never filled originally or someone had the heater run out of fuel. ( VW never make mistakes do they!
) Try to find that priming setting and it will be a lot easier.
The fact that the CAF couldn't be started could mean that the motor is fubarred or maybe dodgy connection. Using the Webasto software you would get the fault reported, usually motor short circuit or open circuit and you can test the motor operation - rpm and duration. You can also test things like the coolant pump function, vehicle fan etc.
It is a pity that I don't have a VW coolant heater to test it's responses to WTT ( the Webasto software), I have VW Airtop heaters from T5s which I can operate fully with WTT ( and they can only be operated normally via the vans systems) and I can't see that the T6 Evo coolant heater will be any different. ( I also have as a test unit a Volvo/Renault Truck Evo AirTop 2000 that can only be operated by the lorries system which can be operated by WTT)
I have always steered clear of the coolant heaters as my hands are full enough with the air heaters but have kept my eyes open for a cheap Evo coolant heater from a T6 for playing with.
Nice to see that there is further help with VCP - we will all end up with so many dongles for this, that and the other we will lose track of what each ones limitations are.