1) Dang, my teleconverter is in my car so I can't compare model numbers, but the one I have is definitely 1.75x and it is the Canon model specifically for the G3.
2) I doubt it, but I can't say with authority. It is not just the lens mount, but also the distance from the main lens which is important. Chances are unless it is not specifically targetted at the G3, it either won't work or won't give the specified magnification. Someone can correct me if I am wrong, though.
3) I haven't tried any others besides the Canon lenses, so I can't really say either, but I wouldn't scoff at the extra .25 magnification. That would be noticable, and one of the things which always gives you the chance for better compositions is to be able to get tighter shots at a distance.
4) This particular lens I don't think will work with the G3 since the thread is too small. They advertise 28mm and 37mm mounts, while the G3 I believe is either 52mm or 58mm. There may be others out there which have the correct threading or are at least adaptable. Given that the link you provide seems to indicate that it will work across multiple cameras, it is perhaps likely that there is something else out there that may be for multiple cameras, but with a larger screw base.
5) Yup, there are filters in that diameter. At worst, you could use step-up rings, but that is a pain, is more expensive, and more awkward to change on the fly. I would highly recommend a polarizing filter, which definitely is available at 58 mm. I know of people who use the Cokin filter system with their G3 and are happy with the results (they use the P system, which is the larger of the two). I haven't used anything other than polarizing and neutral density filters myself, so I can't comment personally, but the people I have talked to seem to like the system. This way, you only have to deal with the mount for the filter holder. The filters themselves are just rectangular plates that slide into the holder. I am a bit biased on the point of filters in that the only ones I use are either polarizing or neutral density filters. Many filters out there are gimmicky and result in artificial-looking results. I focus on landscape photography. I am not against filters, its just that I have seen nature put on better shows than any filter can provide, and I like to be able to say in my final output that what you see represents what I saw when taking the photo.
6) As a Canadian myself, I avoid buying from the US because of duty, getting stuff across the border, and until recently the dollar exchange. In general, I have heard nasty stories about unexpectedly high duties and hard to deal with customs brokers. The G3 is a very popular model, and can be found at pretty much any camera shop in Canada worth their salt. Even London Drugs and Future Shop carry them. Chances are you could negotiate a better deal, or at least some extras. I ended up with an extra memory card and carry case thrown in.
Hope this helps!