I my view, to answer your question 1, look at your second question. The "exhale and sink" in itself is harmeless. But HOW deep do you sink, how fast, for how long, and how is your technique and mental state when doing it? That is where the advanced part come in: To prevent damage. Squeeze and BO is two of the major concerns in freediving, so yes, it's an advanced technique for those reasons. But not because of something "magic".
If you exhale completely, you can damage yourself in 2-3 meters! But that is when doing it wrong: Look at this thread for more info about squeeze and exhale btw
http://forums.deeperblue.com/freedi...97623-full-exhale-pool-adjust-real-depth.html
Make sure you always have a freediving knowledgable buddy to monitor you every second of the dive.
2. As far I can understand from the thread there are two main dangers involved. One being the risk of getting a lung squeeze if exhaling to much regarding to the depth and not being used to it. The other one getting a BO because you don’t feel the need to breath in the same way as inhale diving. There are also difficulties with equalizing due to lack of air in the lunges when going deep. Correct?
In general correct, but about the BO risk: You do get strong urge to breath also in FRC. I've had problems getting answer to why BO is more easily happening, but the closest I came was something like: "You got less O2 in lungs and in general in the system, so before CO2 has risen high, you could run out of O2, and the reason for this is that the lungs are better at storing O2 than CO2". (Just writing it now, I realize that the lower CO2 around BO-level will be a factor also because of the Bohr-effect, but nevermind). Many people seem to believe that CO2 is higher on FRC, and that's why DR is kicking in faster, but from asking here on DB, to me the answers point in the direction of a lower CO2 level... but whatever it is, CO2 is at least lower in absolute numbers around the time of BO it seems. CO2 doesn't have the same time to rise, as when on full lung. In short: It's my impression that the FRC physiology is not very well understood (if I remember correctly
http://forums.deeperblue.com/general-freediving/96075-whats-going-exhale-frc-bo-co2-safety.html NOTE that the graph in the 3. post is not correct! however, but I can't edit and write that now )...
About the third question I don't have a recommendation, depends on what the purpose is with doing the excercise...