Is cataract curable at last stage?

Yes. Sometimes late stage cataracts hide other vision problems in the retina (no view), and thus after removing the cataract, the vision often improves but occasionally doesn't since reveals another problem.
Yes. Although cataracts can significantly affect your vision, they cause reversable loss of vision. The cataract surgery may become more difficult if you wait to long but they can be successfully removed at any time. Your final vision should be the same regardless of how advanced the cataract has become.
Yes. Cataract surgery (CS) is #1 cause of world blindness & most common surgery performed in world & US. Complication risk very low; best time in history to have CS. If no underlying retinal, optic nerve, brain issues, >90% achieve 20/20 vision after surgery; may still need glasses (depends on implant choice); curable at even advanced stages; more info:
Yes. Cataract is a clouding of the lens. As long as there is not other problems with the eye, the cataract can be removed at the earliest stages as well as the latest stages. But there is risk of glaucoma with waiting, as well the cataract surgery may be more difficult to perform at later stages increasing the chance of complications. It can still be curable.
Depends. First: when you say "cure" do you mean treatable, or reversible? Very few cataracts are reversible. If a cataract is ignored and gets very dense, as long as the rest of the eye is normal, then surgical correction will usually yield a good result. If the cataract in part is advanced because the eye is unhealthy, then removal might not give such a good visual result. So it 'depends'.
Yes, in healthy eye. As long as the eye is otherwise healthy, and the cataract is the only problem, then severe, dense cataracts can be removed with various methods, even if the standard phacoemulsification cannot be performed. Pre-operative measurements with ultrasound can often reveal if there is another problem behind the cataract.
Yes. It is very rare for the most advanced technique, with the most modern machines and instruments to fail in removal of even the most dense, late stage cataract.
Generally yes. The most common technique used today (phacoemulsification) may not be possible if your cataract is very dense, but another technique is available and the ultimate vision could still be very good if the eye is otherwise healthy.