A lot has been written in this thread about the current, and how it could not have been over about 3-4 knots. I believe it could have been higher. We have one person who observed the 4 knot current on the surface. Where bottom features would tend to channel the water, and where two different currents come together (as was stated in the original post) the current could have been greater than 4 knows.
Some have said that this greater current would tear the diver apart. No, it wouldn't. It would, however, make manouvering and small motions very difficult. I was Fin Swimming Director for the Underwater Society of America, and the fastest fin swimmer in the world has the record at just under 15 seconds for 50 meters. That's swimming at 3.33 meters per second, which is 6.4 knots (7.4 mph). That's fast, the fastest human in the water! But it won't in and of itself hurt a person. What it does is impose restrictions on what a diver can do in the water.
I've dived in rivers for over 25 years. I dive under 3 foot rapids in these rivers, and am very familiar with current diving. There is a lot that can be applied to this accident too from what I have learned about diving in current.
I noticed that the divers who used reef hooks were at an angle to the current. This provides a lot of resistance to the water, and therefore more force on the diver and the line (and the anchor--coral). While it allows them to look around, it also causes their body to create eddies in various places, and puts unusual forces on the equipment that the divers are not used to.
Current itself is not a problem. It is trying to stop in a current that is the problem. I have been tethered to the bottom in the North Umpqua River to photograph redsided shiners while they were mating in about a 4-5 knot current (it was pretty swift). I chose a 1/4 inch nylon line, and had it attached to my BC with squeeze locks you now find on backpacks. I was diving solo at the time, and accomplished this with no problems whatsoever. So the tethering can be done safely, even without support.
Many times, regular equipment is not matched well to diving in current. Large masks will leak, or be lost, much more easily thas very small volume, close-fitting masks, for instance. Also, the more equipment hanging from the diver, the more resistance to the water.
Many have discussed the "rock climbing" techniques in caves with a lot of current, without telling people why they are effective. The bottom, or in caves the sides of the caves, provide a resistance and set up turbulance that decreases the current very close to the structure. This means that, by going down to the bottom when hooked into the reef, the diver could have taken a lot of the force off the line. I regularly dived under very, very fast water and find enough turbulance on the bottom to make headway upstream. This is a skill that needs to be practiced though, and is highly discouraged on coral reefs because it damages the reef. But, when a life is at stake...
Finally, in regard to the current, the act of trying to look up, to arch one's back, or even to access one's chest will cause a tethered body to plane in the current, usually up or to the side. It will also create more force against the line she was trying to unclip. The better response would be to streamline by putting her head down, planing to the bottom, and taking the stress off the line in that manner. But, again that takes practice, and this was her first time tethered in high current (I think).
Safety Measures: familiarization with current diving, and matching the equipment to the current dive.
