How can one pray with complete devotion, and does reciting the Quran during prayer without tartil and tajwid contribute to satanic whispering? And is a worshipper's thinking, while standing, about the Day of Judgment and imagining himself prostrating before God considered a deviation from devotion?

Reverence (Khushu') is the spirit and foundation of prayer. To attain it, one should seek refuge in Allah from Satan, abandon whatever distracts the mind, abstain from sins, frequently remember Allah, associate with the righteous, and reflect on the insignificance of this worldly life. Reciting the Quran with proper tajwid (elocution) aids in contemplation and presence of heart. Reflecting on the Afterlife also helps in attaining reverence, as stated in the Prophet's (PBUH) saying: "Remember death in your prayer, for if a man remembers death in his prayer, he is more likely to perfect his prayer," and "When you stand for prayer, pray as if it is your last prayer."