What is the ruling on making up missed prayers and fasts when their number is unknown? And is sincere repentance sufficient along with increasing supererogatory acts? And is fasting Ramadan valid if one abandons prayer? And what is the most cautious approach for one's responsibility?

We advise you to hasten to sincere repentance and to maintain your prayers, for abandoning them is a grave sin. If the opinion that making up the prayers is obligatory stands as a barrier between you and repentance, you may follow the opinion of those who rule that making up the prayers is not obligatory, provided you increase your supererogatory prayers. However, it is more cautious to make up the missed prayers. As for fasting, you must make up all the days you missed. If you do not know their exact number, then make up what will give you certainty that your obligation has been fulfilled. Expiation (kaffarah (definition)) is not required of you except for breaking the fast by sexual intercourse. Strive in obedience, increase your good deeds, accompany the righteous, attend gatherings of knowledge and remembrance, and make abundant supplications; for sincere repentance erases what came before it.