The 1/2 mile for helicopter takeoffs only applies to airfields which have no takeoff minima established, or from other locations. Operations specifications often allow takeoff with lower visibility. For instance, we can take off with visibility equal to the landing minimums for any approach to the airport, or with a properly trained and experienced crew, with 'adequate visual reference'. This means, in effect, that I can depart any airport with 1/4 mile visibility in the daytime, and from any lighted runway at night with the same. Heliports, onshore and offshore, require 1/2 mile, and 300' and 1 mile at night.
As for the approach, yes, if you descend to 100' and don't have anything in the runway environment in sight, which includes any of the list of items listed, then it's a missed approach. I have never had that happen, and I've flown many approaches with 1/4 mile visibility reported. At 100', on the GS, you're much less than 1/4 mile from the runway. Note that to fly the approach, only the visibility is required; the ceiling is immaterial at that point. And after the FAF, the approach can be continued even if a new report indicates the visibility to be below minimums, but you still have to have the required visibility to land. This applies to everyone, not just helicopters.
I'm not sure why you can't open the link, because it works for me, but if you start at
http://ecfr.gpoaccess.gov/ and then follow the links through Title 14 to Part 91, you'll find it at 91.175, Takeoff and Landing Under IFR. The language is typically convoluted and bureaucratic, but if you assume that anything not specifically prohibited is allowed, and read it a few times, you'll see what I referrred to.
All this is FAA only, and I make no claims whatsoever for any other country.