The lower voltage you're seeing during pre purge is down to the mutual inductance between the motor and solenoid, so my understanding is. It is common to see around 50v during this time so my mentors told me.
If the coil drew in a screwdriver than that says the coil is energising with a sufficient magnetic force.
If you don't hear the distinctive click of the solenoid/stem at the time its supposed to then yes it would suspect them stem has seized based on the above.
Stems are cheap enough, I would also have a spare coil at hand, from what you've said I dont suspect the coil is damaged but it can go in your van stock if nothing else.
If a stem is partly stuck or fully stuck and the coil is energised for too long the faulty stem can burn the coil out. Basically in a steady state and sound stem/coil the coil is able to dissipate the heat generated from the flow of current, if the stem is faulty this affects the amount of magnetic flux passing through the coil windings and in turn affects the back emf, meaning a dodgy stem allows more current to pass and ultimately burn the coil out. Again based on your words above I doubt the coil is damaged but a spare can go in as van stock, if you don't already have some.
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Sorry I just re read your last post. I though you said coil does see 230v. What voltage do you have when signal should be sent to coil?