Is it permissible to return to smoking again to help apply a diet system, knowing that there is severe knee pain that prevents praying while standing and affects the ability to go to the mosque?

You must remain firm in abstaining from smoking, and it is not permissible to return to it. Weight gain is not an excuse for that. You must also thank Allah for His guidance in enabling you to quit it, and the resolve not to return to the sin is a condition for repentance. As for praying while sitting for one who is unable to stand, it is permissible, but it is not permissible to abandon congregational prayer for that reason, for Allah is more deserving of awe than people. It is advised to be patient in obeying Allah, to fast frequently, and to continue with the diet, for in that there is relief, comfort, and happiness in this world and the Hereafter.