Chris, I am working in pictures I promise.
OK, now the bar I have does not have the bar behind the back seat for the same reason. My Son sits back there. If I had to do it again I probably would do the 6PT one and then not put in the harness bar to help further stiffen the vert. Afterall the subframes helped HUGE...
The 4 (or 6) point with the harness bar, the harness bar bolts on and is therefore removable. Since the main hoop is one peice I would think you can do this in the car.
Safety: I think it would hold the car up in the event of a roll over. Like I said my buddy races at one of the local raceways here and he knows safety. he said that it was well made. It is definitely thick wall tubing and it is a one piece main hoop with good sized floor plates. I don't think it is going anywhere in the event of a rollover.
Originally I got mine because my wife wouldn't allow my son in the car without it (due to lack of a structure over him). I bought this because of her (& him). I have grown to like the style of it. It doesn't really intrude on the front seats at all and it kinda hleps the back seat by giving something to grab onto.
The only downside that I have seen with this is that the main hoop is like 2" (roughly) tubing and it sits an inch or so under the vert top so it is easy to bump my sons head putting him in and taking him out of the back seat. And where it is located even if he climbs in himself he sometimes hits his head.
On the other hand, as I have said already I wish I bought the 6-point one. Maybe one day I'll upgrade.