So your basic question seems to be: Surge or Promote? This is a rather strange question as those two summoner spells fit very differently into the current Meta-game. In fact, I don't believe there is ever a hero I could imagine having to choose between those particular spells as they serve very different roles. So instead, let's assume that you want to know when these two spells are appropriate (and not make a choice between them).
Surge
Surge is a rather recent introduction into LoL and is still having its place in the meta-game examined. While it does act as a much needed steroid for a number of champions missing one (eg. Twisted Fate or Teemo), it suffers on champions who require a lot of farm (i.e. most carries). In comparison to spells like Ignite and Exhaust, Surge (while scaling better) just doesn't guarantee kills the same way through early/mid-game. Now, it's true that Dominion has a shortened early/mid-game, but it's just as true that it has a shortened late game (where this spell really shines). As a result Surge is frequently not the best choice for champions (even carries).
Promote
Promote is another spell which hasn't yet found a good place in the overall meta. However, having been around longer, there are more obvious cases where people use it (i.e. in Dominion going bottom against a Champ who has trouble clearing lanes early on, like Tryndamere or Yorick). Given Promote's role in taking down towers, it's no real surprise you see it more often in Dominion than Summoner's Rift. Usually, you want Promote to push your creep wave into a tower allowing for a better engagement at a node. This frequently means you find Promote on Champions who have trouble clearing lanes or who are good initiators. A Champion that requires more farm or is main DPS (i.e. a carry) is a less likely place to find promote as it doesn't fit their role well.
Garrison
I'm surprised you're reading a lot of information that Garrison is a poor choice; it is quite a good spell. The problem with Garrison is that it really only shines is defending nodes in Dominion. Very frequently people don't do this (usually because it's less exciting). However, there are a large number of places where your team can really benefit from Garrison (solo bot, or even defending top). Garrison can break a channel (from further range), stop a wave push, or even heal a node, all very useful things in Dominion. Even if it only forces the other team to break off an engagement then it is directly translating into victory points (something few other spells do).
When to get it? When you're running a 3 Node defensive strategy or against a team with a lot of floaters (e.g. Rammus)
When to avoid? When you're filling a more important role (i.e. Carry) or you're going for a more aggressive strategy (i.e. running floaters)
Flash vs Ghost
Both of these are excellent spells (and there are even times when you might get both), but in Summoner's Rift, at least, the current meta-game has stabilized on Flash being invaluable. This is largely due to two factors:
- Flash lets you get out of (or into) range of abilities and champions
- Flash lets you "jump" over the twisting terrain in SR
However, in Dominion both of these reasons are less effective. The terrain in the Crystal Scar has fewer features you can "jump" with Flash, the faster game play means champion deaths are less impactful (thus saving, less important). While still a great spell, Flash is nowhere as dominant in the Crystal Scar as it is in Summoner's Rift. Instead, two other features dominate The Crystal Scar which make Ghost a frequently more picked spell:
- Non-linear attack routes
- Contested towers
Having multiple paths to engage (and less vision) often means you are forced to move to positions very quickly. This is the reason champions like Rammus are so frequently banned. What's more, it's frequently a strategy to rush to the top tower at the opening of the game to assert the defender's advantage. In a rush strategy (fairly common) it is necessary that most of the top defenders have Ghost to out pace the other team. As a result Ghost may not enjoy the same dominance that Flash has in SR, but is still a powerful spell (and a frequent choice).
When to get it? Both Flash and Ghost can be great spells on any Champion
When to avoid? Only when you have something which is much higher priority or more synergistic with your strategy (like Zombie Karthas was).
You're right. The best way to use Zhonya's Hourglass is by using it in a teamfight. Lightning Rush in, try to hit as many targets as you can, then activate Electrical Surge, then Slicing Maelstrom. Here is the description for Kennen's ultimate, along with the description for Zhonya's hourglass, from the League of Legends wikia page.
Kennen summons a magical storm that deals magic damage to a random enemy champion near Kennen every 0.5/0.4/0.33 seconds. The storm hits several times, increasing with level, and cannot hit the same target more than 3 times. A Mark of the Storm is applied to enemies each time they are hit.
Unique: Places your champion into stasis for 2 seconds, rendering you invulnerable and untargetable but unable to take any actions. 90 second cooldown.
- At level 6, You can get four Marks of the Storm.
- At level 11, You can get five Marks of the Storm.
- At level 16, You can get six Marks of the Storm.
Add this to your two marks of the storm on each opponent with your E-W combo, along with any Marks of the Storm added from your ultimate before using Zhonya's hourglass. You can potentially stun all opponents.
Since Kennen's ult is very powerful due to its utility, enemies will try to focus you down to avoid stuns. That's when you want to Zhonya's. When it's active, you will continue to stun enemies.
Best Answer
I decided to write my own answer on top of Zamual due to the fact that his answer deals with what the benefits of using Promote rather than the actual question being asked: "When do you optimally use Promote?"
First and foremost, Promote should not be used whenever it is on cooldown. This will most likely lead to you pushing your lane way too much and getting ganked. Or at the very least prevent your jungler from ganking your lane.
It SHOULD however be used when:
This is all assuming you are not playing Dominion. If you are on dominion, you SHOULD be using promote on recharge. Mostly because there is no downside to pushing really hard at all AND the fact that the games usually last a lot shorter so you want to get as much use out of it as possible. For the most part usually the person pushing bot will optimize this the most.
Generally champions with high mobility or assassins should not touch promote. Promote are for defensive and slow champions. Champions like Heimerdinger can use this better because they usually stay in lane and set up a "base" with his turret and the super minion. Champions who are perhaps able to heal or buff the promoted minion can also get the most out of it. I think Janna would be a good choice. Since as a support she has passive movement buff AND her tornadoes are great to escape with, she can replace flash with promote. On top of that, her shield can add additional damage on to the promoted minion.