The bicarbonate content is going to be the most helpful number to have when predicting and manipulating mash pH. You are going to need a water analysis done if you are on a well, or see if you can get a report from your municipality if your on city water. Ward Labs is usually the preferred place to send a water sample. They have a “brewers package” water profile that is reasonably priced.
Maybe on #1, although I’ve never seen it. As to #2, attenuation is pretty much dependent on the fermentability of the wort. Severely underpitching may result in less than planned attenuation, but I’ve never seen higher pitching rates lead to increased attenuation.
IIRC, I have read (can’t remember where) that drastically over pitching can sometimes lead to under-attenuation because of the sheer number of yeast already present in the pitch don’t allow for enough fresh, new yeast cells to grow and the unhealthy ones don’t fully finish the job. I think its pretty hard to over pitch on the home-brew level though.
Denny… Right on, I knew I was going out on a limb with those hypotheses - taking a moment at work - and basically scratching my head a bit. The issue of getting too hot too fast due to a high pitch I recall reading about and logically it would occur more easily for someone without good temp control. i.e. a factor that could make it that much easier for a ferment to get away from the zymurgist in the early stages. I know that I have read especially from Jamil about the importance not only of a sufficient pitch, but also problems from overpitching. Hence the desire to pitch “just right”.
Anyway it all arose at my mis-reading the OP, who meant simply that he makes more yeast than needed and doesn’t pitch it all.
So again the issue at hand might be tied to first, need for better control of mash temps, and bland or soapy flavor very possibly at least in part due to water chemistry and pH, and I don’t recall if he efficiently treats for chlorine/chloramines.
I know that once I successfully negotiated the use of Bru’n Water (including selecting/creating the right water profiles) a lot of problems just went away and my beer definitely improved.