Mono Red Madness is racing you with burn and incidental pings from Voldaren Epicure and Kessig Flamebreather, so life total matters a lot. Healer of the Glade's 3 life on entry is a clean, cheap way to offset several turns of pings and buy time for your graveyard plan. Mesmeric Fiend only strips one card from a deck full of burn and it dies to their removal, giving the card straight back, so it comes out. Masked Vandal can still snag a Blood token off Voldaren Epicure, but that's a low-impact target and exiling your own graveyard creature to get it works against your Dread Return plan, so a couple copies are cut for Undergrowth Leopard's cheap, resilient body.

Mono-Blue Terror's Tolarian Terror and Cryptic Serpent get cheaper for every instant/sorcery in their graveyard, so Faerie Macabre's discard ability to exile up to two graveyard cards can meaningfully raise the cost of their finishers or strip fuel before they cast Dread Return-style plans of their own. Mesmeric Fiend is risky against a deck full of cheap counterspells like Counterspell and Spell Pierce that can strand it or bounce back the exiled card if it's answered, and Masked Vandal has no artifact/enchantment targets here, so both come out. Undergrowth Leopard adds a cheap, resilient body to help push through their tempo plan.

Tron wants to assemble Urza's Tower/Mine/Power Plant and deploy Bramble Wurm, Writhing Chrysalis-style threats and artifacts like Giant's Boulder and Bonder's Ornament, so a second Masked Vandal helps answer their key artifacts once you have a creature in your graveyard to exile. Mesmeric Fiend is weak since it only removes one card from a deck with heavy card selection like Ancient Stirrings and Expedition Map to find replacements, so it comes out. Healer of the Glade buys time and offsets early damage from Nyxborn Hydra while your graveyard engine assembles Balustrade Spy and Dread Return; trimming a Wall of Roots is safe since your curve doesn't need as much mana against a slower deck.