Hi, this is a pretty good guide. There are a several points I disagree with you on, though.
Midding as Heimerdinger is kind of a waste. He gains nothing from the extra experience, and is just as viable in a twos lane. You are better off giving the lane to an Ashe or Twisted Fate, to be honest. This is especially true in your build, as you don't even take Upgrade! at level six.
Midding is especially dangerous with the tactics you suggested--pushing the tower hard and early. Any competent team will call for a gank on mid if you're pushed up too far, which you will be. Your turrets will not be able to save you versus a dedicated gank, and you'll lose your positioning.
I believe Heimerdinger is best served in a side lane where he's able to push more effectively with a partner, but you need to be careful of robbing all the last hits from your partner, so be careful with your turret placement. By going on a side lane, you can effectively stop ganks before they happen with a turret that's placed slightly up the river, in range of the creeps still. Since you basically have a sight ward, you're free to push and fall back as you need. You can't do that in mid.
Finally, I'm really hesitant to follow your suggestion for getting a Mejai's Soulstealer. The only way I'd get this is if I know that the game's going to be an absolute landslide. Snowball items generally aren't as useful in games where the opposing team is well organized and knows what they're doing.