Am I understanding it correctly that if a device's IRQ setting line has a negative number at the end it is running in MSI mode?
Yep.
Is that RSS-not-working-because-of-not-supporting-MSI answer technical nonsense?
May be. There is a need of kinda link between MSI and RSS in NDIS driver, so the questions is: are Intel cut driver for desktop adapters or just nevermind to finish it properly?