Does prayer become invalid if the worshipper feels that he is performing movements without intention or devotion, or if he intended (to pray) Fajr then felt that his intention changed to Sunnah and he did not revert to the intention of Fajr before the Takbir?

The cure for obsessive whispers (waswas) is to ignore them and pay no attention to them. If you are unable to do so, it is because you have not truly striven against your own self (jihad al-nafs), and Allah has promised guidance to those who strive. The feeling that ignoring these whispers makes the prayer seem like an act of pretense is a deception from Satan. The prayer free from whispers is what Allah and His Messenger have commanded, and in it, you will find the sweetness and delight of prayer. Even if your prayer consists only of outward actions, it is still valid and sufficient. With continued adherence to this treatment, you will achieve the desired humility (khushu'). You should say the opening takbir (takbirat al-ihram) once and proceed with the prayer without hesitation or stopping to bring forth the intention, and pay no mind to what Satan whispers to you. This is the treatment, and if you abandon this struggle, this hardship will not leave you.