RE: WHY DOES it require to have engine offset?
If the firewall has a right offset... the engine must be offset to the right side (looking from the front) irrespective of engine mounting position or type of engine ... or the cowl won't line up perfectly...
However, the cowl can usually be moved around a little to accomodate a non offset engine... although lots of larger scale planes 25% and upwards have the offset built into the cowl... so the engine MUST be offset.
Larry