I have a backup taken from my clients sps to my domain so i can
actually start working on that site locally. Is this possible?
If yes, how can i do it?

Do i need to have to utilise the same AD to login or can i use my AD to
login?

Also is it possible to run a excel spreadsheet to use an offline .cub
file to use in pivot tables?

Is there anyway i can utilise any of the webpart currently available in
MOSS to display the cubes content without using excel services?