Is there some in-game source in Skyrim where we learn about Titus Mede and the whole debacle of him assuming the Imperial Throne? Or was that from a novel or something?

Titus Mede I: More information about him can be find in the The Infernal City.

Unless you're talking about Titus Mede II. The only thing I've found about him so far was the book about the Great War.

There's a bit about him in game too. His niece(?) is getting married, and he's involved in the dark brotherhood storyline.

Ugh...I really wish they'd just put all the info in the games. Do they really expect everyone to have read The Infernal City? Not that I hate reading, I read all the time...I just don't like the idea of some author effing around with the Elder Scrolls universe. Even if the book was commissioned/sanctioned by Bethesda...

They're good books.

Horrendously edited, and the guy isn't great at coherent fight scenes, but good books.

The easy, and free, solution is to check TIL's lore notes. We've got http://imperial-library.info/content/infernal-city-lore-notes http://imperial-library.info/content/lord-souls-lore-notes.

There really wasn't a lot of information about Titus Mede in either books - some hints about how he took the throne and ensuing political implications, though. And, of course, he makes a few appearances.

No matter what I do, I simply can't make sense of why he did what he did with Attrebus.

He was a man who could rise to the greatest of challenges, was CREEEEEEEEEEEEED militarily, and brought together a shattered empire. The only thing he wasn't good at, or ready for, was being a dad.

Is that some kind of trailer for a Ted Danson film?

That is one way you could interpret Titus Mede's life as the emperor.