The 6000 year figure was arrived at by picking a verifiable date, IIRC Herod's death. Since Jesus could not have been born after Herod's death, which was in 4BC, Ussher used that as a key milestone, and then used accounts from the Biblical genealogies to work backwards.
The Creation week was taken as literal days for several reasons, one most prominent is that the Jews themselves accepted them as literal days in their calendar system.
It was actually a rather remarkable feat, and one many other people also attempted coming up with different results based on assumptions, changes in various calendar systems and Canons.
Aslan is not a
Tame Lion