According to Carlovac Townway's book series:

- Molag Bal twirled his moustache and destroyed Gildervale
- Almalexia and Sotha Sil fought and banished Mehrunes Dagon, Mournhold was reduced to ruins (and indeed, there is a statue of Almalexia impaling Mehrunes Dagon in Mournhold)
- Sotha Sil traveled to Oblivion on Artaeum to make a 'pact' with Daedric Princes
(-Tsaesci are yuan-ti)

Thing is, Oblivion introduced the concept of the Divine Covenant between Akatosh and Alessia, which was supposed to keep out Daedric Princes already several centuries before the First Age ended (and actually, what is up with being able to summon lesser Daedra when there is a Divine Covenant?).

So, my question, did the forementioned event really happen and if yes, how are they explained by the lore? Or was Townway merely using poetic license?

They definitely happened, though maybe not exactly like the book describes. The covenant is not nearly as perfect as Oblivion makes it out to be.

There is a barrier between Mundus and Oblivion.

Its working can be described by three rules and some exceptions.

• Mortals can pass the barrier, see Doors to Oblivion.
  
• The Barrier keeps Daedra out, see words of the Clan Mother, Trials of St. Alessia, Amulet of Kings, Liminal Bridges and so on.
  
  • Unless Barrier is not sufficiently strong. For example the end of the Oblivion crisis.
    
  • Unless Daedra are sumoned. For example temporary summons or long term summons of small Daedra or until 2920 the Princes.
    
  • Unless Barrier is pierced. For example Daedric gates, which might be used in long term summons, see Darkest Darkness, and Liminal Bridges.
    
  • Unless extreme manifestation of a Daedric sphere occurs in Nirn. For example see the Seven Fights of the Aldudagga
    
  • Unless it can be avoided by secret means. For example Azura, see Words of the Clan Mother and the Story of Morrowind.
• Lorkhan can't pass the Barrier without any known exception, see Words of the Clan Mother.

Not implied, but mentioned for clarity:

• Innert materials are not Daedra and can enter.
  
• Enchanted items are not Daedra and can enter.
  
• Voices are not Daedra and can enter.

Further more the barrier should not be confused with Sotha Sils pact of which it is implied that:

• Daedric Princes will not heed the summoning calls of the lay-man, see 2920 for further information.