Does the prayer of one who moved to salute before completing the Tashahhud, then returned to complete it and salute, become void? And is it obligatory to repeat the prayers in which the same thing was done out of ignorance? And how is the number of these prayers determined?

If someone forgets the tashahhud and says the taslim, then returns to it, their prayer is not invalidated, and they must perform the prostration of forgetfulness (sujud al-sahw). As for repeating this in previous prayers, there is no need for you to repeat them. If doubt becomes frequent to the point of becoming a Waswas (obsessive whispering), then it should be ignored.