Is it obligatory to repeat ablution and prayer for someone who forgot to comb his beard with water (takhlīl al-liḥyah) and remembered it during prayer? And is one who exaggerates in combing his dense beard with water to ensure the water reaches considered to be acting contrary to the Sunnah?

A thin beard must be run through with water (takhleel) during ablution (wudu'). As for a thick beard, it must be run through with water only during the ritual bath (ghusl), but not during ablution. Running water through it three times is legislated, based on what was reported from 'Uthman (may Allah be pleased with him) that he ran water through his beard three times, in imitation of the Prophet (peace and blessings of Allah be upon him). If one doubts the number of times they ran water through it, they should rely on certainty, building upon the lesser number. What hangs down from the beard must be washed.