Raz, the losses in coaxial cable(regardless of whether it's 18ga RG6 or thinner)at audio frequencies are so low that they don't become significant until hundreds of feet are involved. If it's more convenient for you to splice, there's no problem.
As far as sub positioning, don't take the "every room is different" thing as a given. Your room dimensions are quite good from the standpoint of modes and the general rule for such a room is that both the strongest and smoothest response is likely to be had by positioning it directly in the corner which is farthest away from an opening.
