Is the prayer valid with frequent inattentiveness and movement within it, by abandoning 'ṭuma'nīnah' (tranquility) then returning to regain it and performing 'sujūd as-sahw' (prostration of forgetfulness)?

We advise you to turn away from whisperings (waswas) and not pay attention to them. Tranquility is achieved when the limbs remain still for a short period, and you most likely used to perform it. Therefore, there is no need for you to return (to that movement) with the prostration of forgetfulness (sujud al-sahw). If you had abandoned tranquility, then you must perform it and the second prostration after it. If you did not perform the second prostration at all, or after a long delay, then your prayer is invalid and you must repeat it.