SafeT, your picts (and the vid recomended by gasman) are too big for me to download on my dial-up, but Ive been fooling with a lot of leaners since Katrina so I'll offer up 2 cents. Fenceposting works well, especially using salami cuts so that the leaner doesn't settle back on your saw. But at some point as the leaner approaches verticle you may need to get some pull on it, to pull the top out so that you are not standing in the line of fall when it comes loose from the tree it is leaning on. Winch (on my jeep) or Tirfor is my favorite, but truck or tractor, block for change of direction if necessary, whatever, dont be under the canopy when it comes loose. When to stop cutting and start pulling is sometimes a question answered only by hard won experience.
Also, leaners tend to scrape a lot of bark and break a lot of branches off of the tree you are trying to save as they settle after each "fencepost" cut. I'm partial to climbing first (while I still have a root ball on the leaner to steady it), cutting out the canopy of the leaner down to the point of the hangup, fenceposting until the stem is light enough to pull, and then pulling either from the butt or from the top according to conditions. Even if I plan to pull from the butt, I always install a bull rope on the leaner just below the hang up point before I come down and start fence posting, so I keep a "control from the top" option available during the fence posting.
It can be risky taking out the canopy (leaner can roll, branches can springpole, hangers can fall on you when you cut supporting branches, supporting tree can give way, on and on) so you have to be very careful, but the risks can be mitigated by:
(1) being sure the supporting tree is firm or tie in to neighboring trees for your climbing (2) tie the leaner to the supporting tree at the hangup point before you start trimming the crown of the leaner so it doesnt roll out on you while you are in it and (3) stay out of the line of fire as you releave the energy on spring pole branches.
Hope this helps. Many of the pros that watch this site may be able to offer more. If you are not a pro, and safety really is first, finding a pro to hire as coach or to do the work for you may be money well spent. These things can be very much more dangerous than is readily apparent.