Ok, I've seen so many Heimer guides say you NEED clarity. I used to use it, loved it, but it's not the best for several reasons.

Clarity does not help in the middle of team fights (maybe rarely) but it won't net kills, or slow down tanks, or kill DPS. It also does not increase your survivability. It just gives you more mana to push, which can be accomplished through MP/5 runes, items, and golem buff.

What does work REALLY REALLY well, andI highly encourage all the Heimer fans out there to try is Exhaust. If you are getting ganked, put it on their DPS and watch him miss as your turrets eat him alive. It useful to catching runners. It works wonders in team fights where everyone and their mother say Heimer is useless. It completly shuts down their normal damg DPS for several critical seconds, usually changing the tide of a 5v5 team battle.

Anyhoo, my 2 cents

Exhaust sounds to be quite the interesting option. While this certainly does warrant some trial, it sounds as it exhaust would be the same thing as a second grenade as well as UPGRADE!!!, which already fill their roles adequately to my experience. And while exhaust certainly would help with chasing where grenade and UPGRADE!!! wouldn't, the role of chasing I feel should be left to chasers rather than a pesky Heimer. As such, exhaust would certainly hold viability, but from first examination seems to be quite situational.

Clarity, on the other hand, is always useful when Heimer does not have the golem buff, which cannot always be counted upon. Items, I feel, do not outlast the longest of sieges, and mana regen runes would help, but that would be taking away valuable slots for the all-important magic penetration.

And of course, my guide is not one that says clarity is needed, but rather the summoner spell to replace if one favours escape utility.

All-in-all, exhaust sounds to be a summoner spell with areas of superiority just as any other, but from first scrutiny, I still prefer clarity for its support in sieging, the role most substantial to Heimerdinger.

DMG1: The entirety of my experience as Heimerdinger, as well as my guide, is on Summoner's Rift, so I have no credibility in regards to commenting on TT strategy.

As for heal— In a laning situation, heal is a spell that contributes to sustainability in regards to harassment, which Heimerdingers should be able to properly avoid with practice. It could be used as emergency health in situations in which Heimer's being ganked, but a spell that offers an escape mechanism would excel in such a situation.
In team fights, the measly amount that heal gives to allies, as well as the range you would have to be in for it to apply to teammates, would leave heal to quite more of a situational state. An escape mechanism, still, I feel, would provide Heimer with a more effective choice overall.

The Stark's aura only affects allied champions, unlike, say, Aegis of the Legion, which applies to all allies, including creeps.