Remember what you said about a perfect fort location, with an underground river, a chasm, magma pipe, flux, and different biomes in each corner, including a desert, glacier, forest, and mountain?
I FOUND ONE!!!
And better yet, I can't give you world parameters since I forgot to export them before I embarked :-P
Alas, the map is pretty big - 8x8 at least to get much of everything, and probably 8x12 if you want the HFS.
The trick is to generate worlds with all the varience parameters pretty much maxed out, and with a giant temperature range - -50 to 150 is what I used. This allows for a fortress map with both freezing and scorching biomes. If you minimize the vertical dimension of the map, it works all the better. Also, set the elevation to something like 280 - 320, that will give a lot of squares on the mountain threshold. Finally, set the weights to things like rainfall and volcanism to favor bath extremes - there's lots of flux in 0-20 volcanism, so if you set the weights high on both ends, but low in the middle, you will get a lot of areas where 0-20 volcanism borders 80-100. Note that you'll have to set edge oceans and peaks both to zero, since you won't get either with these elevations.
Finally, I hope you enjoy searching for the right spot. I exported map images and searghed the detailed map in paint shop - with the grid turned on and set to 16x16 - in order to find spots with the correct biomes. The site finder simply doesn't help if you want a site with high AND low temperatures, for instance.