1 - I use 3/32" (1.8mm) Spyderline from Annapolis Performance Sailing. It's really light and rated to 350 lbs. It's also listed as "low-stretch", which testing bears out.
2 - I've measured less than 3/4" stretch, under my weight (165 lbs.), in my ~ 10' ridgeline. I can remeasure for accuracy if anyone's interested. I've tried 550 cord, and I'd estimate that it stretches WAY more than just 10%. Just an eyeball judgment, though.
3 - I'm not really sure how you're measuring stretch using a plumb bob. I measured the line with no load, then hung, got in, and measured again.
I've run through the force diagram for a ridgeline before, but I don't have it handy right now. If you want, I'll sketch it out again really quick and post it. I do remember that, under 'normal' conditions, I came up with ~ 250 lbs. breaking strength as being save for both ridgeline and ridge biners. By 'normal' conditions, I mean my weight and a sag angle of about 20* (I think). That being said, I've been using unrated biners on the ridgelines of both my homemade Speer-type and my HH ULB with no visible stress to the biners.
The HH ridgeline is probably a lot stronger than you think. I'm assuming they source it from wherever they get their support Spectra (who knows where that is...). I know the ridge is Spectra, and going from the pretty impressive strength (through testing) of the support lines, I'd bet the stuff on the ridge is pretty strong, too. I have no idea where they get either cord, though, so it's kinda hard to say.
"Physics is the only true science. All else is stamp collecting." - J. J. Thompson