Unfortunately, you only get that one chance to buy a Death Flower. You'll need to get another one, since it's one of the required ingredients for making ambrosia, which is clearly the end goal of the event.
The easiest method is to look in the gallery for a lot that includes either a Death Flower or a Death Flower plant. There are lots of them.
You can get a Death Flower through gardening by grafting an Orchid shoot to a Pomegranate tree or a Pomegranate shoot to an Orchid plant. I don't know which way is better, except that Orchids have two seasons (Winter and Spring) but Pomegranates only have one (Winter). You should probably plant your first Death Flower so you can easily get more, but it also only grows in Winter. Be sure to stock up while they're in season or grow them indoors.
There's a chance of getting Death Flowers from a seed packet, probably Rare Flowers.
I think there's also a small chance of getting them by asking Patchy the Scarecrow for seeds (Seasons).
You don't have enough points for the final reward because you are missing either the Paranormal stuff pack or the Realm of Magic game pack. Week 1 included a task that uses the seance table from Paranormal and Week 3 included a task to visit the Magic World. These tasks only show up if you have the required pack but without them, you simply don't have enough points to unlock this week's last reward, so it will have to wait until next week. The reward you're missing right now is just the Fishing Vol 2 skill book. You can already buy it from a bookcase or a computer if you don't want to wait for next week.
There are supposed to be enough points to unlock all the rewards by the end of the event according to EA_Cade , but the last 5 rewards are already available from either gardening (Cow Plant, Trash Fruit, Death Flower) or the Rewards Store (Want Refresher potion and Re-Traiting Potion).
You can't go back and repeat a quest after it's finished, as far as I know. That's why quitting and reloading didn't work.
I think that's everything you asked about. I hope some of this helps you or at least makes you feel a little better about it. Good luck.
