that was ok till you say disconnect and reconnect facia
the main power supply will generate low voltages to power facia board but it could be either
I tend to think display board as disconncting power or disconnecting display board do the same thing and it powers up.
if the main board had failed if you disconnected facia i would not expect it to come back on
but can never say for certain
you could try and find the voltages from the lower board and see if it is still there after failure
but at the end of the day I would quote for both boards and offer to order display first to see if it works. Make it clear they still pay for the whole job and both boards but you can reduce price if it does
as a very long shot disconnect motor and heater and see if anything changes
Dave