Are the prayers in which the letter "seen" is mispronounced—pronounced somewhere between a "mufakhkham seen" (heavy 's') and a "muraqqaq sad" (light 's'), without a hiss or a whistle—valid or not?

A Muslim should not pay attention to the whispers of Satan, especially the whispers concerning the articulation points of letters, because they divert a Muslim from the proper etiquette of prayer and distract him from understanding the recitation. The imams disliked extremism and exaggeration in pronouncing letters, and that is not from the Sunnah (definition) of the Prophet, peace and blessings be upon him. Therefore, it is sufficient for you to pronounce the letter 'seen' according to your ability without artificiality, and your prayer is valid, and you are not required to repeat it.