How do we reconcile the vastness of God Almighty’s mercy, in which we have no doubt, with His holding us accountable for simple matters, such as “delaying prayer” and “lengthening one’s garment,” with severe punishments?

The reference for determining whether a sin is major or minor is the Sharia. Just as we are commanded to believe in the vastness of Allah's mercy, we are also commanded to believe in the severity of His punishment.

It is out of Allah's mercy towards His servants that He warns and admonishes them to avoid what displeases Him. Another manifestation of His mercy is granting sinners a reprieve to repent. Allah's mercy has overcome His wrath, as stated in the Hadith Qudsi: "Indeed, My mercy has overcome My wrath."

Sins do not necessarily lead to entry into Hellfire except when ten preventing factors are absent, which include: repentance, seeking forgiveness, expiating good deeds, atoning calamities, the intercession of the Prophet (peace be upon him) and others, the supplication of believers, what is dedicated to the deceased, the trial of the grave, and the terrors of the Day of Judgment.

Delaying prayer is not punishable unless it is delayed without an excuse beyond its entire prescribed time, so that it becomes a make-up prayer. As for unintentionally lengthening one's garment, there is no accountability for it, and the proof is the Messenger of Allah's (peace be upon him) saying to Abu Bakr: "You do not do that out of pride."

As for trailing one's garment in general, scholars have differed on it, and the preponderant view with us is its prohibition, even if not intended out of pride.