Is a worshipper's beautifying of his voice and prolonging of his prayer, due to imagining his loved ones watching him, considered showing off (riya')? And is riya' possible without the presence of people? And how should one act if Satan causes him to doubt his deed?

We advise you to continue your worship and to repel the thought of showing off (riya'). It does not harm you if it occurs after you have started an act and you push it away. Disregard the whispers of Satan, which aim to deter you from worship, and proceed with your worship, sincerely devoted to Allah the Almighty. Be careful not to abandon worship for fear of showing off, and strive to achieve sincerity, seeking help from Allah. Riya' is performing an action for people to see.