Decay was very strong in the BG build. It's harder with the 5c low-land version, but I think it's still worth it.
Dredge and TES often run it too, despite low land count. The main reason is decks like that (and this) can't afford to run too many removal slots without diluting the core engine and really need the first copy to kill the thing you want to kill. Missing sets you far back. Uncounterable is very strong. Also, Decay hits almost anything. With most others, you have to choose between a Disenchant or creature removal, and you could draw the wrong one (if they've diversified their hate like they should).
FoV is a nice new tool. The problem is your green count is low to support it (and it's counterable). Decay increases the green count making FoV better. With Decay I have 15 MD green even without Troll, while you just have 13, and there aren't many other green cards worth playing.
Not mandatory, but in my experience it helped add value to the card. It beats Decay and Fatal Push. It lets you block immediately, especially OTD. It stops Bolt without being forced to discard (holding back cards can be useful for bluffing or playing around hate, especially postboard) and without having 4 creatures (with just 3 they can respond to one activation to kill 4/3 Troll). You lose to StP either way, but if they use StP on Troll then Vengevine and Amalgam just got better. StP is torn between too many targets.
Not the only reason, but it was useful. My original builds of this deck 8ish years ago ran
Carrion Feeder +
Gravecrawler +
Lotleth Troll, so there was a significant zombie subtheme. The higher zombie count made Gravecrawler easy to recur. Gravecrawler could be double cast in one turn (sac via Feeder or Therapy) to trigger Vengevine even with no hand and bad topdecks, recovering from disruption, providing 2nd and 3rd winds. Gravecrawler was also a grow combo with Feeder. Feeder gave a sac outlet to protect recurring creatures from StP and Terminus, keep counters off Jitte, and swarm around Batterskull lifelink (Stoneblade was much more common then). Both Feeder and Troll use the recurring creatures to be significant Grow threats, aside from just being enablers. Beatdown plan B was strong even through graveyard hate. DRS provided incredible mana boosting, fighting through Wasteland, allowing Troll regeneration, hardcast Vengevine, faster Carrion-Crawler growth, and turn 2 Buried Alive. There was a lot of synergy between the pieces, and they transformed well into a grindy fair deck postboard.
Now with Hollow One and without DRS, the 5c build seems less synergy-oriented and more linear and explosive: discard outlets and free creatures. Troll shone more with its synergies, grindiness, and plan B utility. As just a discard outlet it's weaker than the others. That makes me question if it still is necessary.
Relying on drawing Petal + Troll is a 2-card combo. You shouldn't need to get lucky for your backup engine. The backup engine is supposed to bail you out of bad hands.
My idea was that OUAT + London Mulligan + Faithless Looting should help dig into the 8 copies of PImp/Tribe. Your older build had 10 copies to consistently have an outlet, but that was pre-OUAT pre-London.
Looting doesn't just discard 2 cards. It also digs for more cards, something none of the others do. That should get more gas and more mana, while still pitching 2 cards for value.
Another 2 cmc outlet to consider is the new
Seasoned Hallowblade.
Zombie Infestation is a bit awkward requiring double discard, but it also lets you pitch noncreatures and provides lasting value via multiple bodies.
I never liked the card disadvantage of Petal. It seemed less important without Lotleth Troll. Most things cost 1 mana. Dredge runs off 12-14 lands. Looting and OUAT should help dig into more lands, something the previous build couldn't do. In theory, that should be enough to compensate, but it needs testing. I might need to increase the land count. I have a 15th land in the SB (Karakas) and considered a 16th.