Is the state of devoutness in congregational prayer and the absence of intrusive thoughts when realizing a missed raka'a due to fear of people seeing, while suffering from obsessive-compulsive thoughts when praying alone, considered a form of showing off (riyaa')?

What you mentioned is not showing off (riya'). Rather, we advise you to maintain congregational prayer as long as it removes these whispers (waswas). Know that the optimal cure for whispers is to ignore them and not pay attention to them. So, do not repeat the Takbir or anything else, nor repeat the recitation of any verses merely because you imagine a mistake. Instead, strive against yourself to get rid of these whispers.