An 'In-Line Fig 8' is another name for a 'Lazy 8'. I'm not familiar with any industrial testing of the knot but I would imagine, from the way the rope sits, it would be about as strong as an 'Alpine Butterfly' and maybe slightly less than a standard 'Fig 8'. They are great for single point re-belays and ensure no abnormal loading of the knot if the re-belay anchor fails.
I've seen some interesting rigging over the years but simple is usually best. 'Double Fig 8 on the Bight' ('Bunny Knot' with the common loop sitting on TOP), when the anchors are close together and a 'Fig 8 on the Bight' or 'Fig 9 on the Bight' and an 'Alpine Butterfly' when the anchors are a bit further apart. I always tie 'Fig 9's' as they are usually easier to untie than 'Fig 8's', unless I'm rethreading a 'Fig 8' around something. 'Fig 9's' are a little bit stronger than 'Fig 8's' though the difference is negligible unless knots are properly dressed and set, which they rarely are.
I know 'Double Bowlines' are regularly used for rigging, particularly in France, but again, unless they are properly dressed and set they immediately deform under load and cannot easily be visually examined. I remember following a team down Nettle Pot in Derbyshire. I arrived at the top of Elizabeth as the last man had just set off down. Looking at the knot, what the knot-tyer probably thought was a 'Double Bowline', I could see it slipping quite a lot whenever the abseiler bounced during his descent. I never did work out how it had been tied, it wasn't in the Ashley Book Of Knots. I re-rigged it with a 'Fig 9' and and 'Alpine'. There was some testing shown on this forum about the potential consequences of clipping a cow's tail into the loops of a 'Double Bowline'. The very fact that it is necessary to tie a 'safety' stopper knot behind the 'Bowline' should suggest the knot is unsuitable in favour of a more suitable knot that is stronger, easier to tie, easy to visually inspect and easy to untie after use, e.g. 'Fig 8 on the Bight' or a 'Fig 9 on the Bight'.
When it comes to knots, the BMC do a great job of endorsing what in industry would be regarded as bad practice. I work at the Glastonbury Festival every year running the free climbing wall in the Kidz Field. There is a mixture of cavers and climbers on the teams and every year when we have our first safety briefing there is usually another BMC recommendation about how we attach the harness to the rope.
If I were risk assessing the attachment method there would be no question in my choice, a double or triple twisted 'Scaffold Knot' with a triple action connector. This would ensure the carabiner cannot be cross loaded and 5 year olds fingers cannot undo the carabiner accidentally.
Unfortunately the procedures the company who supply the wall follow say they always follow the recommendations of the BMC so its a screw gate carabiner attached loosely in a 'Fig 8' loop, with a 'Half Double Fisherman' stopper knot. This year the stopper knot was only tied around the tail of the knot (around itself). The year before the stopper knot was tied around both itself and the 'Live' part of the rope.
Apparently the change came about following someone clipping into the gap between the main knot and the stopper knot. I can't remember the detail of what happened next but CIC's on here should know about it.
The issue of a stopper knot in addition to a 'Fig 8' has been discussed at length in industry. The consensus is that there is absolutely no need for it. As far as I am aware there has never been a reported case of a 'Fig 8' unravelling itself under load but there has been at least 1 accident because a stopper knot was used.
I think the BMC advice on joining ropes suggests tying the 2 ends with a simple 'Overhand Knot'. Dynamic rope is generally a lot softer than low stretch rope and so you will probably get away with the knot staying firm if you pull the ends apart but if you did that with anything other than new low stretch rope it is likely the knot would unravel if pulled apart with any significant force. I've witnessed that test, its not a method I would recommend with low stretch rope, particularly if it still feels new.
Mark