Dusk I’ll attempt an answer (from “idiot” to “idiot”)…
TLDR answers to your questions in order: no; yes; no.
All the research/guidelines I have seen (or seen summarised) point to the fact that there is a relatively broad range of acceptable values even in terms of ‘summary’ parameters (overall hardness and alkalinity); more so if one starts considering other factors (e.g. corrosiveness of sulphates, chlorides, hydroxides, potential biological effects of organic ions like citrate, solubility and safety of various salts vs. use convenience, protection from corrosion because of passivation deposits, …).
On the other hand, there is a set of imponderable/subjective/personal preferences on taste. I have experimented recently (and only on Turkish brew) with four water recipes back-to-back: carbon filtered, “hard” tap water (GH =294 and KH = 236 as per Thames Water report); de-ionised not remineralised; de-ionised remineralised with KHCO3 and MgSO4 (GH = 60, KH = 40); de-ionised remineralised with MgSO4 and Ca citrate (GH = 70, KH = 50). There are easily visible differences in e.g. how much foaming the coffee produces, and there are also easily perceivable differences in taste. The best tasting for me is the Mg + KHCO3 recipe - but that’s obviously subjective, and the preference may well change if I changed coffee or brewing method.
There is also another type of subjective preference: how much is one ready to accept in terms of scaling and/or corrosion in the name of better taste… or the hope of it. That in turn introduces even more variables in terms of materials (copper, steel (which steel?), brass, aluminium, platings, linings), technology/machine design and brewing preferences.
What water to choose at the supermarket - very simplistically, one that has a low amount of dissolved minerals, particularly Ca and Mg (as those are the main culprits for causing scale). A low mineralisation water may not have sufficient alkalinity, although that is easily corrected by adding a little bicarbonate (as per the thread above) - however, KH is seldom indicated on the label of bottled waters, so one is left guessing - or buying a KH kit. Then, coffee made from a low mineralisation water may not have the best taste (subjectively…), so one ends up adding Mg or Ca. But that is more likely to cause scale, especially if one has used bicarbonate to add alkalinity. Never ending circle of despair.
Or pick up a bottle of de-ionised water, use Dr. Pavlis’s recipe (50 mg NaHCO3/l) and be done with it. Ingredients are cheap and available in pretty much any supermarket. It won’t scale, and by many accounts it tastes pretty nice when making espresso.