Marley was Scrooge’s *business* partner and you can argue that Scrooge not bothering to change the sign was because he was too much of a miser to do so. And the reason he was the sole mourner was because both of them were ruthless businessmen and he went out of obligation/reading of the will. Also, his happiness of seeing the boys and old guy friend from the past are because they’re nostalgic memories to him which reignited a spark of Christmas joy in him (the whole point of that chapter). And he gets mad at the ghost because when he was upset over seeing his ex happily married, it basically said “hey, don’t shoot the messenger”.