Okay so I did some testing in a new game, tried a few different scenarios out and this is what happened.
Test 1:
Put my sim on the dynasty plot in Riverview and covered it in spawners and a couple of swift grow stations, had her read till level 7 gardening then picked up seven seeds and identified them, two were death seeds, I then used testing cheats and starved her to death twice holding just one seed at a time, both times she gave Grim the seed.
some screenshots
Death seedsDead sim with seed in inventoryGrim with flowerTest 2:
This time I had her pick up 10 special seeds and didn't identify them, I left them stacked in her inventory and starved her again, I was hoping at least one would be a death seed, well Grim arrived and whacked her with his scythe and that was that, but before he left the lot I had time to drag her seeds out of inventory onto the ground.
Once the change household message appeared I picked Meadow Carpenter-Rhodes, she's level 5 gardening so after a trip to the library to get to level 7 I had her collect and identify the seeds, one was a death seed, so unsure if he would only take two or if it was something else I did some tests with Meadow.
Test 3 & 4:
Meadow had the seed from my sim and also identified two more, they were all stacked in her inventory, she gave all 3 to Grim, she then collected 11 seeds left them stacked and didn't identify them, she died, so if it's unidentified and stacked they wont give it, if it's identified yet stacked they will (proof in next test)
Test 5 & 6:
I switched household again for some seed roulette, first I resurrected my sim and Meadow via philosophers stone and genie then collected Meadows seeds, she had been carrying a death seed in her stack of 11, so now time to test if unidentified single seeds worked like my dynasty sim had.
seven sims and seven unidentified seeds plus my sim to identify them, all seven sims died, no death seeds in that batch.
reloaded ditched those seeds and collected seven more, saved then identified them, one death seed, reloaded and played another game of seed roulette, six died only one left and I thought i'd made a mistake but then number seven gave grim a deathflower.
So there you have it, it would seem a single unstacked seed will do the trick, can't say if it changed with the patch or not, I almost feel it did as somebody would have had this happen surely?
I feel you should try it yourself though as it will probably change the rules of keeping seeds in inventory if this can happen.