All very good questions!
Yes the test tubes being plastic do go cloudy and fine scratches, this scatters the light source so reading can go off, Replacements tubes are cheap enough, drop me an email and we can sort that out. I did some hunting around for optical quality glass test tubes that would fit but at £75 for 3, I thought I doubt it!
The "ideal" levels, I know the book contains lots of helpful advice but that isn't one of them, well not if you have a vinyl pool.
Alkalinity in the form of bicarbonate in water is used as a buffer solution. This is to try and keep the water at the correct pH which was originally done to protect concrete/plaster/tiled pool surfaces from degradation by water in the pool. This doesn't happen in a vinyl pool, the bicarbonate of soda solution in a pool buffers the pH at 8.3, way above what we want. The higher the Alkalinity the stronger the "Pull" to pH 8.3 and the more PH- you need to get it to move.
This is great news for the pool industry, they sell more bicarb (sorry Alkalinity+) and then more pH- so you can keep adding each in turn what fun, not.
In a vinyl pool you need only enough Alkalinity to stop the pH drifting from 7.2-7.8 if your pH is relatively stable then it's fine, out of interest what is the level? Mines about 37ppm.
Be warned if you use low Alkalinity as I do it takes a lot less pH- to get the pH to move. Multi action galets could cause the pH to drop slightly using low Alkalinity.
Calcium hardness, some of this is desirable, Calcium being a metal has a + charge and that helps to flocculate - charged particles in the water and then it helps the filter to collect those (Only if your filter is fine enough) otherwise it just goes around and around so nothing to worry about in a vinyl pool.
Hope that helps