Phil's set up sounds similar to mine, only I'd tie your end line off with a 'Scaffold Knot' (AKA Half Double Fishermans Noose Knot /forum/images/graemlins/crazy.gif /forum/images/graemlins/grin.gif); the karabiner can rotate and side load with the Bowline.
A quick way to tie the scaffold knot is here:
http://www.treemettlenexus.com/training_assessment.html
The best thing about clipping into a ring sideways like that, is the barrell of the karabiner can't get punched through if you swing off a branch into the main stem. I was involved in an accident investigation where a branch snapped the guy was standing on, as he pulled down on the rope to take in slack. He flew into the trunk with speed and force - A peg connected with the gate of the Krab and pushed it back and open. The Bowline had hooked under the barrel on the gate and flew out. The guy stopped 15 ft off the ground as his chainsaw cuaght in a fork. The branch snapped, he hit the deck, and was injured by the falling branch and chainsaw.
This can also be avoided by rotating the krab so the gate faces the climber, but this can catch the shirt when the gate shuts on autolocks, preventing full strength closure. The krab needs to be choked with the lifeline, to prevent it turning.
The failed karabiner was a Petzl ball lock by the way - the plastic gate doesn't completely cover the nose of the krab, and so is easily forced aside. The lower part of the gate also deformed. I use these krabs else where though.
Choose krabs with thick gates to help prevent this.