someone please tell me I'm wrong (if I am ), but hardness and calcium are the same thing. there are a number of cal/hardness increases out there. I don't have vinyl pool, so can't comment on whether the 100 level is good or not.

the cya question is one that most here will say is personal preference. the SWG manufacturers call for pretty high levels. Seems like most with sWGs shoot for 50 or so. But I'll let others chime in on their opinion there. i would, however, strongly suggest that you experiment with your CYA, slowly creeping up to a level that works for you. If you overshoot, tough to go back down, as you know.

It takes higher levels of chlorine (whether from SWG, bleach, pucks, granules, etc) to both sanitize and shock as your CYA level increases. Chlorine doesn't stick around long in a pool with low CYA. There are a number of people here on this forum that operate high CYA pools without issue. I don't remember hearing too many with low CYA that can say the same thing, for what it is worth.

with my poolpilot, i have 45-50 CYA, shoot for about 4ppm FC, and would shock up around 15 if i needed to. but again, that's what works for me.

as for the difference in salt tests, SWG units tyically have a calibration setting on them to correct. I know that mine was off about 400. the question really becomes which test do you trust. there are some known issues with Ben's salt test kit (there is a sticky on it somewhere) involving drop sizes, I believe. I would look for that, read up on it, correct the issue (if there is one) and go from there. I state this because your contractor and pool store where close. obviously, if all three of your sources are in the same ballpark, you've got your answer.