Any sense of how much helium will permeate through the grain structure of the drive enclosure during its lifetime?
You might have a few drives with bad seals that fail early. But say the majority are sealed properly, but the helium still leaks out at x small rate over time, you might have almost all the drives suddenly fail at around the same time after say 6 years or whatever.
I don't think you can really know if they're truly reliable in that sense until you can take one that's been sitting one in a drawer for 10 years and plug it in.
A high-altitude environment might put more pressure on the seals than other places, that could shorten the life-span, but other than that it shouldn't be a huge deal. Most drives have a commercial life-span of no more than 4-6 years anyway. After that you're on borrowed time.
Helium also goes through whatever the sealing gasket is made from, at a much higher rate. http://lpc1.clpccd.cc.ca.us/lpc/tswain/permeation.pdf is a neat chart, probably the gasket is buna-n because it's cheap. I think that chart and https://en.wikipedia.org/wiki/Permeation are enough to get an order of magnitude answer to OP's question, but I'm not quite up to doing the math.
Serious vacuum systems have to worry about this sort of thing more than one might expect. I once had a project where two test engineers and myself spent around a week hunting for leaks in a vacuum system that turned out to be caused by my specifying Silicone o-rings instead of Viton.
No. Helium hard drives are laser-welded shut. See http://www.seagate.com/files/www-content/product-content/ent... for the details about what parts are made out of what and the basic details of fabrication.
I'm not sure why I thought there was a rubber seal, I have vague memories of seeing an exploded view with one somewhere.