Need to remove chimney below attic floor, then somehow connect a 10" at a 45 and run back out for the roof

Assuming this is a masonry chimney: No can do.

You will either need to reinstall a masonry chimney with an offset made to your specifications or line the entire chimney with an approved system from the chimney connector up and through the roof.

What kind of chimney? Is this for a dfireplace, furnace or what, is it lined?