I will only refer to the method I describe in my "over packing" video.
Personally, I have never had an entire session fail using this method, however...
You CAN still overheat the bowl. I've had this happen a number of times. One thing I failed to mention in the video (actually it got edited out in the interest of time), is that once I get it smoking well, I remove the wind cover. Every time I've had a burnt taste it was because I left the wind cover on too long. When that happens, I remove the coals completely and let things cools down.
I only use ******* Hookah natural coals, so I can't comment on the Exoticas specifically, but maybe two are too many. With the *******s, on the bowl in the video, two works well to get things started. On occasion I've had to reduce to one for a bit and then put the second one back on.
On a side note, yesterday I used the same method with a mod bowl that I have. It is a little bigger in diameter then the bowl in the video but about the same depth. I used Nakhla Sweet Melon and 3 CH naturals. It was the absolute best session I've had to date with this method. In fact it was near flawless. The session lasted two sets of coals and the bowl was cashed out completely when I was done.
Bottom line, it sounds like a heat management issue. Either that or the bowl wasn't over-packed enough, allowing some air space between the foil and the shisha and thus causing it too actually burn.
Hope this helps. I'm still feel like I haven't quite nailed this method down, I'm still experimenting and learning myself!