Hot damn that probably cuts better than most of the knives in my kitchen, thats a real piece of work. That would take a long time to make, as you have noted our knife need not be as good. So lets find another rock, one that is heavy and strong, and knape it into a knife shape and find some dead wood and attach it to the dead wood with the same kind of plants that we used to make the belt. After that we can use the knife as our butchering tool, and use the stone in order to make a sharper tip for our spear. I think an atlatl is beyond our comprehension, even if we could make one our guy just couldn't make the leap from spear to atlatl without any simpler ranged weapons in between.
A good spear is more valuable than a good knife, you can use it to kill food and if you ever really need a sharp edge you can use the tip of the spear. And if you really need a knife you can break your spear and then have a knife. A knife is more versatile but the things a spear can do for you are more important than the things a average knife can do for you. Regardless we are making a knife first because we are not hungry for now and so don't need to catch food for a while, and the knife will allow us to make our spear, and butcher the corpses better. Then we will be able to protect ourselves with the hides of our fallen enemies.
So here is what I believe we should do
>Find 3 rocks, one rock that is hard to knap other rocks into tool, and two sharp rocks, one for the knife and one that we can attach to the spear later.
>Collect some of the plants we made the belt out of, in order to attach the knife to the handle.
>Find a dead peice of wood for the handle.
>Go back to the cave, knap the rock that will become the knife, attach the knife to the handle, knap rock that will become the spear tip, attach to spear.
Complete the first three tasks before going back to the cave, once you have returned to the cave feel free to sleep and then knap, or knap then sleep.