MacOS – Can’t Boot from Snow Leopard anymore after having installed mavericks on an external HDD

There should be a point—after you choose the language, but before it starts trying to install the OS—where you see a Utilities option in the menu bar.

Do you get to this point? If so, what happens when you choose Disk Utility?

There's more information about this at Using Disk Utility to Repair a Disk (University of Wisconsin).

OK, now it's a different question, so here's my different answer:

If repairing your main partition is what you want to accomplish, why not just run Disk Utility off the 10.5 system disk?

I'm not sure if this will work in your case. But I have a 2010 MacBook Pro I cannot install the retail copy of Snow Leopard onto. That retail disc is 10.6.3, and the laptop needs a higher version.

To overcome this, I installed Snow Leopard on an external drive connected to an older Mac. I then updated Snow Leopard to 10.6.8 and copied the contents of that drive back to my MacBook Pro via SuperDuper.

Now, in your case, I'm not sure if Snow Leopard will work at all on the recent MacBook Pro. But, if you have access to an older Mac, give my solution a try.