I love this concept, the idea of snake sapience is interesting. But to your points
Evolution
The luring snake is probably a good place to start, this evolution will take a long time, but snakes have been around for a long time; so this should work.
- The spider tailed viper is a good place to start, perhaps we now say that a new snake replaces this one, and the spider tailed viper has to move, it keeps moving until it arrives in the Congo or the Amazon. The jungle seems to breed intelligence.
- When in the jungle, perhaps the snake has to work harder to find food, ambushing and luring become its sole hunting strategy and it likely will lose its venom glands.
- It is at this point we work on its tail, now as ridiculous as it is we have two choices here
Tarzan approach, my first idea was that the fringes evolve to move in order to better climb the canopy and this over a couple thousand generations leads to a prehensile tail-hand thing. But this felt like I was stretching reality. So I think the next path is more likely
Venus fly trap approach, Just as it sounds, first the fringes evolve muscles at the base to quickly close to trap small critters, then eventually the muscles around them become more independent of each other and eventually through more small changes until you get a hand like appendage.
Tool use
I don't imagine they would be good at this, you need two grasping appendages to make things, so you will need to replaces the jaws free form jaw with a real one, a big change that is kind of ridiculous. This or the have to rely on working together with other snakes to build this; which does help with sapience; personally I'd suggest that, but the mouth also works. As to the kind of tools, that is unpredictable because snakes have not been shown to use tools, so we cannot know what kinds of tool they would build; the sky is the limit.
