Boot OS X from external HDD

Internal HDD on my MacBook (alimunium unibody) crashed. Had a back up on an external HDD. But now it wont boot from the external. Tried the usual recommended tips to boot OS X from external HDD including;
Hold down Option
Hold down Command+Option+Shift+Delete
Hold down Command+R (for internet recovery)

Neither of them worked. I was running Snow Leopard. Questions are;
a) when exactly are you meant to "hold down" the above mentioned keys
b) which of the above combination is the best to boot from external HDD (on which i have backed up using non-automated Time Machine)
c) is there any other commands that are more correct
d) is the internet recovery option doable for Snow Leopard users (i intend to upgrade to Lion anyways so is it possible for my machine to connect directly to the Mac App Store and download and install it) or is this feature for either new Macs and Lion users (already purchased) only

Any help would be highly appreciated!

First you must have a bootable disk to boot from an external. A Time Machine backup is not bootable. If TM is what you were using to backup, you are not going to boot from that.

If you were running SL, there is no internet recovery. This is a brand new option with Lion.

Press the Power button, press and hold the option key until you get to the boot menu. That will show you any drive you have connected to the machine that is bootable.

From what you've told us, you need to insert your SL disc, boot while holding down the option key, select the SL disc to boot from and reinstall.

Before any of that though, what's wrong with the internal drive... did you try booting from the SL disc and repairing the drive... have you replaced the drive already... etc.

Took the MacBook to the Apple Store and they booted it from their boot disk successfully and thus concluded that it must be my HDD thats the problem. Unfortunately the MacBook is no longer under warranty so they wouldn't do anything about it.

The HDD however was a later installed one (since the original one too crashed 3 months ago) and therefore is still under warranty and will be replaced with a brand new one (probably going to opt for a SSD instead this time to avoid further hassle plus much improved performance).

The backups were indeed mere Time Machine ones so thanks for clearing up as to WHY it wouldn't boot from the external HDD. Guess next time I'm better off making a bootable copy rather than a mere TM backup (although I'm sure thats a lot more complicated but thats a discussion for later).

For now, I find myself in a soup since I don't have the SL disc with me anymore. Is there ANY way around this? I do have the original Leopard disc that I got with my MacBook but can one upgrade to Lion from that? Also if i were to fresh install Leopard on the new SSD would I be able to restore from backup even though the backup was made running SL whereas after fresh install I'll be running Leopard?

I may be able to obtain a friends SL disc which he'd purchased as a family pack. I used that the last time when my original HDD crashed. Does each time one installs the OS X on a new HDD even if same computer one of the five uses of the disc get used? Cause in that case his disc may be of no use to me.

And while we're on the topic of Mac OS X and upgrades; is it possible to install Lion on a new and blank HDD (that is one that has NO previous OS) if one decides to buy the OS X Lion USB Thumb Drive from the Apple Store? Or is it still a requirement to first install Snow Leopard before upgrading to Lion (even if the thumb drive is being used for installation)?