On the GH/KH test kit...you count the drops until the water in the tube first changes to the colour. Sometimes this is not much of a change, but that is the point it changes. Holding the tube uncapped vertically over a sheet of white paper can make the colour easier to see than holding it sideways.
Second point, on the sand...only if the sand is calcareous will it affect parameters. Sand such as crushed coral and/or aragonite or rift lake cichlid sand is calcareous and it will raise the GH and pH and KH. But ordinary sand such as most non-calcareous aquarium sands, play sands, etc, are inert. The buildup of organics in the sand over time may have the opposite effect of lowering pH, but that is another matter.
Third, your molly will not last long in this water if the GH is only 5 dGH. Mollies must have moderately hard or harder water with a GH above 10 dGH, and must have a basic pH (meaning, above 7.0 and it doesn't matter how high above that). Shimmying is usually a sign of lack of minerals in the water, and the fish cannot compensate so it weakens and dies.
Fourth issue, the water softener. This is likely more trouble for fish, as some of these (if not most) remove calcium and magnesium salts by adding sodium chloride (common or sea salt). This is not good for any freshwater fish, though having said that, mollies would tolerate this better, but the water is still too soft so that is the real issue for mollies and similar harder-water species (all livebearers for example). Test the GH, KH and pH of your water before it goes through the softener if you can; it might be better to use that source water.