@Caterina — I am unaware of the fact that a death flower seed can cheat death with Grim. But, so long as you did not knowingly use some banned cheat, I see no reason for you not to continue.
I am 99% certain I had this happen in my legacy-style game - my sim was attempting to repair a dishwasher after she singed herself trying to repair a stereo, and silly me thought the dishwasher was considered a plumbing object. Um, WRONG! Learned that the hard way, that's for sure!
Anyway, cue the music & husband's freakout, she handed something to Grim, Grim mentioned something about needing to find a lovely pot for this plant and everything went back to normal. She had one less special seed in her inventory, she did have not enough gardening skill to plant them, and I didn't know there was a deathflower in Riverview at the time (I just learned there is one a couple of days ago).
Worked out for me beautifully as I didn't really want to switch gears with Dad and I wanted a choice of heirs (though I suppose that type of ghost baby would've been pretty darn awesome had I fully known about the "oh my ghost!" opp), but I can see how it'd be a problem in a challenge like this. However, I haven't retested the theory and I didn't document it, so I can't provide definitive proof.