Now, it's interesting to think why the expression should be "dead as a doornail." What is there about a doornail that it makes you think of death? You'd think a coffin-nail would be more likely to be considered the deadest piece of hardware in the industry. Still, the wisdom of the aged is in the expression, and it wouldn't be appropriate for the likes of me to question it. So, if you'll pardon me, I'll simply repeat that old Marley that old Marley was as dead as a doornail.
Now, did Scrooge know he was dead? Of course he did! Marley had been business partners for I don't know how