Is the calamity of one who abandons prayer greater than the calamity of Iblis, for his refusal to prostrate to the Lord of Adam, unlike Iblis who refused to prostrate to Adam?

This statement is not correct. Iblis's disobedience by not prostrating to Adam was disobedience to Allah's command, not to Adam's command, and for that reason he disbelieved. Allah Almighty says: ﴿And [mention] when We said to the angels, "Prostrate to Adam"; so they prostrated, except for Iblis. He refused and was arrogant and became of the disbelievers.﴾ His disobedience was coupled with objection and arrogance, which is in greater contradiction to the glorification of Allah. The argument against him was more severe because he witnessed the greatness of Allah in a way that one who abandons prayer has not. This does not diminish the significance of abandoning prayer, for the disbelief of one who abandons prayer is established by the Quran, Sunnah, and the consensus of the Companions, but disbelief has degrees.