Does the abundance of supererogatory prayers increase the chances of dying prostrating? And does observing the obligatory prayers while committing some sins guarantee uttering the Shahada at the time of death?

A good ending is hoped for one who has preserved the five daily prayers in congregation, the regular Sunnah (definition) prayers, Duha (forenoon) prayer, and some of the night prayer (Qiyam (definition) al-Layl). A believer may sin, but he hastens to repent and does not persist in sin, unlike the hypocrite. Allah Almighty says: ﴿Indeed, those who fear Allah – when an impulse from Satan touches them, they remember [Him] and at once they have insight.﴾ When a believer is afflicted by something from Satan, he remembers Allah's punishment and reward, thereby discerning the truth and acting upon it. It is not a condition for a good ending that one dies prostrating. Therefore, strive in acts of obedience, repent to Allah, think well of Him, and supplicate to Allah to grant you the best of endings.