Is repentance from one sin valid while committing other sins, and does delaying prayer due to inability to determine the Qibla or fatigue lead to major disbelief?

Repentance is obligatory for all sins. It is not a condition for the validity of repentance from a specific sin to repent from all sins. If one misses a prayer due to an excuse, the majority of scholars hold that it is obligatory to make it up immediately. One is not sinful for delaying it if the delay is within the prayer's time or due to an excuse. Performing the prayer at any part of its time is sufficient. There is no harm in delaying the prayer from its very beginning to perform it in congregation. Facing the Qibla (definition) is obligatory. If one is unable to inquire about it and exerts effort (ijtihad (definition)) and prays, then their prayer is valid.