What is the ruling on one who doubts that the Last Day is confined to planet Earth only, and is his doubt considered disbelief, despite his repentance thereafter?

What you have mentioned does not warrant doubt about apostasy occurring. It has become clear to us that you are suffering from severe obsessive whispers (waswas) concerning matters of faith and rulings on purity. Therefore, we advise you to turn away from these whispers and not to give in to them, because they are a severe illness, and giving in to them leads to confusion, pathological doubt, distress, and legal difficulty.