All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.
Nothing of what you mentioned leads to doubts or Waswasah (obsessive whispers), so seek refuge with Allah from the accursed satan and place your trust in Allah, for, verily, He is the best to protect and the best to help.
If we consider that everything that historians and biographers said in this regard is true and accurate, then Ibn Katheer underlined in his book Al-Bidaayah wa An-Nihaayah that Seeth, son of Aadam (Adam), lived for a very long time. He said, "Seeth was one hundred and sixty-five years old when his son Enoosh (Enos) was born, and he lived after that for eight hundred and seven years."
So if you come to know that they lived for that long, the problematic aspect in this regard would be removed regardless of whether we believed that the Arabic word Qarn means a century (one hundred years) or a generation.
Ibn Katheer said:
"In Saheeh Al-Bukhaari, Ibn ‘Abbaas narrated, 'Between Aadam and Nooh (Noah), there were ten Quroon (plural of Qarn), during all of which Islam was prevalent.' [Al-Bukhaari] So if what is meant by Qarn is one hundred years - which is the first meaning that comes to the mind of most people - then the period between them would definitely be one thousand years. However, the period between them may be longer than that because Ibn ‘Abbaas specified that those ten Quroon followed the religion of Islam, and this does not negate that there were later Quroon between them during which people were not following Islam. However, the hadeeth narrated by Abu Umaamah indicates that the period (between them) was specifically ten Quroon, and Ibn ‘Abbaas added that they all followed Islam. This refutes the claim of some historians and authors that Qabeel (Cain) and his offspring worshiped fire, and Allah knows best.
If Qarn is intended to mean 'generation', as in the following Quranic verses (which mean):
• {And how many have We destroyed from the Quroon (generations) after Nooh …} [Quran 17:17]
• {Then We produced after them a Qarn (generation) of others.} [Quran 23:31]
• {…and many Quroon between them.} [Quran 25:38]
• {And how many have We destroyed before them of Qarn?...} [Quran 19:98]
And the Prophet, sallallahu ‘alayhi wa sallam, said: 'The best of Quroon (generations) is my Qarn (generation)...'
Then each generation before Nooh used to live for quite a long time. Accordingly, the period between Aadam and Nooh would be thousands of years." [Al-Bidaayah wa An-Nihaayah]
For more benefit, please refer to fatwas 223153 and 224266.
Allah knows best.