Kristina, thanks again for a very interesting and valuable article. It seems completely accurate except for the lack of recognition of how "FRC" diving is used as an every day, all the time, style of diving. There a quite a few divers who use this technique and, I suspect, more who use it and are not aware of it. Granted its a minority of divers and granted it takes a bit of adaption and granted that you won't reach your maximum possible depth while line diving, but it works great for serial diving (in my case down to about 30 m). For many divers, it is much, much better than full lung. You really should research this aspect and comment on it in part 3. There is lots and lots of info in these forums.
A quibble: FRC is really a lousy term(half lung is better) to describe the technique, because there can be so much difference in the amount of air an individuals "FRC" is. My understanding is that, for many divers, FRC is about 40 per cent of full lung, For others (like me) FRC is pretty close to forced exhale empty. It also matters a lot what a diver is wearing. A thick wetsuit, even a well fitting one, reduces FRC. I would not be at all surprised to discover that many divers in wetsuits are diving at about 60 percent of full lung, the same amount of air most half lung divers typically dive with. Your article correctly notes that depth with this technique should be limited by feeling rather than arbitrary depth, which gets around the individual variability difficulty.
but still leaves lots of room for misinterpretation of results between divers.