Is the prayer of one who missed the first congregation and prayed in a second congregation, led by an Imam who prays quickly, valid, given that he recites Al-Fatiha quickly and without concentration?

The Imamate (leadership in prayer) of someone with a valid excuse, such as urinary incontinence ( سلس البول ) or similar conditions, is valid according to the most preponderant opinion. Therefore, there is no harm for you in leading the prayer. The Imam must be deliberate in his prayer, out of consideration for the congregants. Humility (khushu') in prayer is recommended, not obligatory. The prayer of someone who follows his companion is valid, even if he misses out on humility.