I got a 5 point harness, and I mount rear straps to rear seat belt bolts, submarine belt to front seat bolt, right lap belt to right seat belt clicker bolt, and left lap belt to floor, probably not best, but It holds me in place great, with the stock seatbelt I spent most of time on track fighting the seat to stay in place. When I put cage in I will change mounting points. How do you address the spinal compresions issues? To me it seems like that could happen no matter were you mount them.
No, the roll cage is for racing. I know on a mustang it makes them handle alot better, by stiffening chassis, I suspect it will be the same on the focus. And with as many times as I have went off track (usually due to other people) or just hitting a tree or wall, its a very good safety item. Autopower makes both bolt in and weld in kits, or you could have one custom made pretty easily.