Great OP!
1) I don't think I quite understand the mechanics behind Shock Trooper. I mean I get he has trample/haste/lifelink, but its only good for one turn, then hes kind of useless. Am I missing something with him?
2) There are two conflicting opinions about the guildgates. I see both sides. Maybe I will try it out with them in it at first then see what happens.
Again as always you guys give great help.
To understand Spark Trooper you need to understand the joy of Ball Lightning, the original 6/1 hasty trampler. He was around in old-school mono-red lists as a very efficient beater. Thinking of it as "only good for one turn" is the wrong way to look at what is basically a burn spell. If you had a spell that said "1RRW: Deal 6 damage divided as target opponent chooses among that player and any creatures he or she controls. You gain 6 life.", you'd play it in a RW aggro deck, right? Well, that's what Spark Trooper is. The life gain won't always be relevant, but against another aggro deck (or especially another burn deck) that 6 life could make all the difference.
As for your questions...
1) Personally, I'm anti-Guildgate for the same reasons mentioned above. If your curve tops at 4 (and I think it should), you basically never want a land to come into play tapped. However, I would keep the Strongholds, as they let you maintain your tempo with anything you drop late-game.
2) I think the aggro strategy is WAY stronger in the current environment. There's no good mono-white sweeper, so if you want to play control you have to rely on wiping the board through damage, which isn't easy.
3) Ash Zealot is a solid 2-drop, but the other one to consider is Gore-House Chainwalker, since you likely won't be blocking anyways. Legion Loyalist is pretty good but you want to drop and immediately attack, which means you won't put him out on turn 1 anyways. Naya Humans are becoming popular enough that I'd look at Rakdos Cackler or Stromkirk Noble as a real 1-drop.
4) In this build, I'd definitely stick with Firemane Avenger or Hellrider in the 4-drop slot, and not have anything above 4 (not even a Thundermaw). That will let you get away with running 22-23 lands, which will make the deck feel really lean.
Cuts:
Aurelia, the Warleader
Foundry Champion
Cloudshift
Assemble the Legion
Blind Obedience
Additions:
Stromkirk Noble
Ash Zealot
Searing Spear
Legion Loyalist