I'm really not convinced by the whole heir thing, and I don't think its set in stone that Gryffindor's heir would be Dumbledore, its just someones pet theory. As Egan said, it just seems a little unbelieveable that you'd have two of the founders last descendants about at the same time, they've had hundreds of years to die out, why now?!
My thought about Crouch Jr was more to do with Crouch Sr, I figured he wouldn't be too impressed to have a son in Slytherin, (I have nothing to base this on, just my personal feelings and interpretation!!)
Here's a theory that we've been developing on my forum about why voldemort didn't die.
1. Voldemort didn't try the AK curse on Harr. We have no first hand account that he did.
2. He didn't want to kill lily because he was smarter than we think and knew that Lily was going to try some spell that would end in her death.
3. Voldemort is some how related to Lily. So he got a little bit of protection from lily's spell. This also makes Harry an Heir of Slytherin, explains why Voldie chose Harry, and fits in rather nicely with CoS.