I think there's a hierarchy to the kinds of responses that are warranted to situations like the ones in the OP, and it should be more dependent on how the question (demand) is broached than whether the OP feels blunt or not. It's like playing a trump card - you don't slap it down on the first move, you wait and play increasingly more "valuable" cards until you have no choice because the other person isn't accepting what you've already said. Whether the trump card involves more specific personal information is up to the individual, but I could definitely see using it as a last resort to really drive home a point (because I myself would certainly do this if I felt it would be the fastest and most effective way to shut down the conversation).
I can't, I have a conflict at that time.
Sorry, I have a previously scheduled event I cannot change.
I have an important medical appointment that cannot be rescheduled.
(Seriously?) I'm about to undergo <procedure>, I won't be at your meeting.
One would hope the last line wouldn't be necessary, but some people just won't take any other version of no for an answer.
In the second case, I can totally see dropping the "she's dead" bomb if the other person won't take a more vague response on its face. To me it doesn't come across as uncaring about the death, it conveys the point that the other person simply wasn't listening to the responses until they got blunt enough to make an impact. In my head, the conversation would have gone like this:
Coworker: "Well, Susan sure screwed up the database. We need to make sure that doesn't happen again"
OP: "Our team is on it, and we'll have it resolved by X date"
Coworker: "Yes, but we can't risk it happening again. Susan really needs to be retrained"
OP: "Rest assured, our processes will stress the training for that database element"
Coworker: "I want to see your retraining plans for Susan, it's very important that she not mess up again"
OP: "I understand completely, all employees' training will be closely monitored by management"
Coworker: "Susan should have to take some kind of evaluation before she handles the database again!"
OP: "Well, she's dead, so I'm positive she'll never do it again. Here's what we can do to try to stop others from doing it..."
To be fair, the last line could have been "Well she's no longer with the company so I'm positive she'll never do it again" but OP's version was more direct, shorter, and left nothing up for inquiry (since I can imagine the coworker then asking if she had been fired for incompetence and what else might she have screwed up, since the convo was so specific to Susan).
OP - hope your results are good!