Is repentance obligatory for what preceded it, and do supererogatory prayers compensate for missed prayers, or is it obligatory to make up missed prayers, given their accumulation due to obsessive thoughts about purity and not making them up?

You bear no sin for missing prayers under the influence of obsessive-compulsive disorder (OCD), but they remain a debt upon you that must be repaid, for the Prophet, peace and blessings be upon him, said: "The debt owed to Allah is more deserving of being fulfilled." Make up the prayers according to your ability, in a way that does not harm your body or your livelihood. The Malikis permit making up two days' worth of prayers with each day if there are many missed prayers. Making up missed prayers takes precedence over voluntary prayers. You must be wary of and turn away from these intrusive thoughts.