The attachment point at the bottom certainly seems to work, from your data.
It seems (and I may be wrong here) that the two most common methods for ensuring stake set in soft soil are to use a stake designed for that or to place something heavy over the head of the stake.
The "Y" stakes sold by MSR (their Groundhog line) and similar seem to be very popular; I use a set, in sand, and have yet to have a problem with them. Note that I've not done scientific testing on them, though. This is anecdotal.
The other stake I've seen popularized for multi-terrain use is aluminum gutter spikes (gutter nails) from the hardware store. I've not used these personally, so I can't comment upon their holding power.
I'd be very interested to see what the rest of your testing turns up. Thanks for taking the time and effort to do it!