Sounds like that SpritecraftFree.jar is causing some sort of issue. I would suggest simply uninstalling whatever that is, and attempt to uninstall or repair Java using https://java.com/en/download/.
I have built my computer for about 400$, and it can run minecraft and many other games (League, Dota 2, TF2) At a decent 60FPS, so if you're on a budget, and not looking just for a laptop, I would suggest using newegg or something to build a budget gaming desktop.
I dont know know much about command blocks, but I assume teleporting your players after they fall past a certain Y value may be a good way to fix this issue