I'd prefer that the ability does not get the attack below zero because in general units with more hp than attack are weaker than the others and if it can make the attack negativ its another disadvantage for the weaker creatures because dead is way better than negative attack.
The upped version is way to overpowered in my opinion. The effect of the ability is doubled and thats the main reason to have a banshee in the deck (usually the upped version is not worth double the unupped) and the upped banshee itself grants +1/+1, it should stay a 2/2 creature and mourn could be nerfed too (-2/-1 or -1/-2), i dont think it will appear that strong in the game.
Additionally i got a general problem with this card: Like you said it enables
to kill something (except for plague, which is slow) and that may cause
to get to strong because it has so many ways to use dying creatures. In general i prefer
to get units which die itself or kill allied units, that way units die and can be utilized by other cards, rather than being able to kill enemy creatures.