All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Committing an act of disbelief out of ignorance does not necessarily entail that the doer is no longer a Muslim. What any person afflicted with Satanic whisperings should do is to ignore those whisperings completely and pay no attention to them at all because excessive indulgence in them incurs grave evils on him. Therefore, he should disregard such whisperings regardless of how great his doubts are about going out of the fold of Islam. Rather, he should believe with certainty that he is a Muslim who adheres to Tawheed (Islamic monotheism), and all praise be to Allah, the Exalted. What is obligatory on a person who wishes to embrace Islam is only to proclaim the Shahaadah (the Two Testimonies of Faith), and it is not incumbent on him to perform the Prayer of Repentance. As for the obligation of performing Ghusl (ritual bath), scholars held different opinions regarding it. The opinion we regard as preponderant is that Ghusl is not obligatory as long as the person was not in a state of ritual impurity necessitating Ghusl when he apostated and this is the opinion adopted by the Shaafi‘i School of Fiqh.
Since the Prayer of Repentance is not a condition (for conversion to Islam), it is also not a condition to observe Khushoo‘ or rules of Tajweed and the like therein.
A final word: you must ignore satanic whisperings and strive to repel them, for indulgence in them is one of the worst means leading to evil.
Allah Knows best.