Could a spirit or demon be corrupted by the void?

Probably, demons usual invulnerability comes from their spiritual nature (I think). Since the Void is able to consume physical and spiritual things (since it's able to consume souls) demons are probably just as vulnerable as anybody else.

Probably no, The Void is stated to only corrupt organic life and spirits etc don't really come under that category, with some more powerful exceptions.

Spirits are probably the best defense against The Void because you can't disintegrate a spirit, you can't melt a spirit and you can't eat a spir - well technically you can but it'll just come right back out.

A lot of people complain about The Void being the ultimate, unstoppable evil but the truth is The Shadow Isles might well be the best weapon to fight against it.

Or the spirit will corrupt the void Shadow Isles style lol.

Well Nocturne has a Void skin but I'm not sure if that can be considered a yes.

Void consumes everything. I don't think spirits are exceptions when not even cosming beings are.