Ah, an Orbit.
Well, that seat takes a few more steps to install (with LATCH or a seatbelt) than most other seats. Whether you use the belt or LATCH, you still need to twist the knob on the seat.
I have only installed the Orbit three times--all with the seatbelt--and never had a problem. Of course, none of the belts came from that high, but I don't see how that would make a difference. I suppose it could, though.
In your original post, you said that the seatbelt installation isn't "good enough." What is the exact problem you're having? When you check for tightness, you want to check ONLY at the belt path--not at the front of the seat (toward the front of the vehicle). Even the most tightly installed seats can be loosened that way.
Also, like I said before, you just want to give a moderate tug with your non-dominant hand. You don't need to TRY to loosen it--you just want to make sure it's snug. As long as the seat moves less than an inch AT THE BELT PATH, you're good.
Also, when you install, are you putting the belt through the lock-off? (I think there's a lock-off on that seat--it's been a while), and you're turning the tightening knob, right?
It's hard to tell for sure from your photo, but it looks like your seatbelt locks at the latchplate. If that's the case, most seats should install about the same way as they do with LATCH. Buckle and tighten--no need to pull the belt all the way out. It usually helps to pull the belt right next to the buckle instead of on the other side of the seat.