All righty, I'ma give this a try. I'm not too keen on the validity of this (I did the loyalty rewards and got my skills from PvZ 1) but I ASSUME that, if you're after a particular ability, I would purchase -only- packs that focus specifically on the side you're trying to get them for.
As such, for Mega Heal Bomb, I would purchase only the Amazing Brainz Pack (35k Zombie only, 7 items), for Plant specific do Fertilizer Fun (Plants only 35k Pack). Since the special abilities are -VANILLA BASIC SKILLS- these skills will drop from these packs for sure.
I don't know if the 15k Amazing Pack Will provide skills, if it does, then I would consider it over the 35k Pack that provides both Plant and Zombie items.
As always, Rux offers the new Gw2 Skills only (Like the Bling Potato Mine for the cactus! Or the Dark Bean bomb), and as such are prone to his random selectiveness of what he sells. These also cost 500k a piece. (I'm still waiting for Mood Shield for Citron!!!!)
As @Ziago_J mentioned, the majority of the original alternate abilities are available completely at random through the purchase of Sticker Packs; Dark Flower, Solar Flare Beam and Rainbow Heal Beam for the Sunflower, Iron Maiden, Artichoke Drone and Potato Nugget Mine for the Cactus, Super Zombie Stink Cloud and Multi Rocket for the Foot Soldier, Heal Station, Energy Warp and Armored Heal Station for Scientist, Spiky Spkeweed, Super Sticky Goop, Sprint Burrow and Chomp Cannon for Chomper, Shield Decoy, Long Bomb and Ultra Tackle for All Star, Sombrero Bean Bomb, Retro Gatling and Super Pea Jump for the Peashooter
Two alternate abilities, Super Multi-Ball for Super Brainz and Bigger Better Butter for Kernal Corn, are only available through the purchase of the Deluxe version (or Upgrade) for the game.
Several new ones are being slowly made available through Rux during his monthly visits to his Shop; Arcane Lotus for Rose, Super Homing Ball for Super Brainz, Pizzazzling Potato Mine, Red Artichoke and Bling Maiden for Cactus, Mood Shield for Citron, Dark Bean Bomb for Peashooter, Bling Barrel for Captain Deadbeard, Vampire Spikeweed for Chomper, Future Dummy for All Star and Rainbow Heal Flower for the Sunflower are all exclusive to his Shop.
The last alternate ability is the Rainbow Dash which is exclusive to the Unicorn Chomper and which cannot be obtained separately from owning that Character.
Unfortunately there's no means by which to improve your chances outside of purchasing one type of Sticker Pack over another, depending on what you are trying to obtain, because the system is completely randomized and not influenced by anything at all, including it's own past behavior in what it did or did not give you.
