Is it permissible for me to complete my prayer when I start showing off in recitation, then intend to stop this showing off? And am I thereby considered to be mocking the religion, or is this from the whispers of Satan?

To overcome obsessive thoughts (waswas): Seek Allah's help, ignore them, distract yourself, and continue your prayer without paying attention to them. There is no blame upon you for the occurrence of waswas as long as you do not act upon them or speak them, for Allah has forgiven my Ummah for what their hearts whisper to them. Your dislike for these obsessive thoughts and your resistance to them indicate that they do not harm you. There is no mockery in what you mentioned, and you are not required to perform ghusl or renew your Shahadatayn. Turning away from thinking about matters of waswas is the correct approach.