To me it looks like the designers really looked at one of the criticisms of Arkham Horror, the inconsistent storyline, and created a system for that. They may have decided to make it mostly cooperative (Descent/Dracula) because they either couldn't find a card based system that worked as well as a live opponent, or they just plain wanted to make it a little more competitive.
From the early previews and designer diaries, Mansions seems to borrow a little mechanically from different games. The Overlord/Keeper player and the scaling based on the number of investigators (similar to the threat tokens in Descent), the buildable mansion like Betrayal (except not random), the title from the Call of Cthulhu RPG, and mooching characters from Arkham.
I would imagine the investigator figures are unpainted versions from the painted figure line. Similar to FFGs re-using Descent and Runebound figures. The monster figures with the slot in the base would seem to hold the Arkham Horror monster chit. This would also make sense, FFG could re-use the monster tiles and AH fans could use the figures for both games.