Why did the Prophet, peace and blessings be upon him, not clarify that the matter of the one who commits suicide is with Allah instead of describing him as abiding eternally in Hellfire? And what is the reason for the contradiction between the narrations prohibiting the sale of cats and the difference in the narration of "a man found a dog and gave it water" and "a prostitute"?

The revelation of similar texts in the Quran and Sunnah represents divine wisdom, as this worldly life is based on trial and tribulation. Islamic law is part of this cognitive tribulation; for Allah revealed in it clear and firm verses that cannot be changed, and He revealed what opens the door for ijtihad (independent reasoning) to allow for diversity of opinions and to enrich the religion.

Ibn Uthaymeen said that the wisdom behind the Quran's diversity between clear (muhkam) and ambiguous (mutashabih) verses is trial; for if it were all clear, the wisdom of testing would be lost, and if it were all ambiguous, its quality as guidance would be lost. The clear verses are referred to in cases of ambiguity, and the ambiguous verses are a test for the believers, so that Allah may distinguish between those in whose hearts is deviance and those who believe that the entire Quran is truth from Allah.

The hadiths that carry severe warnings, such as the hadith of the suicide, are for cognitive trial and to push scholars to exert effort in understanding them. They also contribute to the emergence of Islamic sciences and their principles. The Lawgiver has stipulated everything that protects from perdition, and the Quran guides to that which is most upright, and the Prophet, peace be upon him, did not leave anything that brings closer to Paradise or distances from Hellfire except that he clarified it.

Another wisdom behind the strong wording of warning hadiths is deterrence and intimidation. The Salaf (pious predecessors) affirmed the texts of warning as they came; because interpreting them away would diminish or weaken their benefit. As for the other examples mentioned in the question, there is no contradiction in them, such as selling cats and watering a dog, for the latter was confirmed for a man and a harlot woman.