On the pH drops I can see that if you failed to dose properly, although I would argue that most situations shouldn't require dosing pH drops.+1 with the pill box idea.... my thought that larger would be more stable is based simply on the fact i believe a larger amount of water would take larger quantity of heat, time, or chemical to change things such as temp and ph rendering them easier to handle... for instance if the PH up bottle reads 10 drops per gallon... that few extra drops if amistake is made should in theory make a more dramatic difference in a 10 gallon vs a 90 ... i believe the same for heating the tank if your heater breaks temp drops should take longer to change in a 90 vs a 10.... i could be wrong, however....
On the heater, your heater should be sized to your tank so a smaller tank should have a smaller heater capable of producing less heat. This breaks down in tiny nano-tanks though as it is hard to appropriately size heaters in the less than 5g tanks.
Why would a dead fish cause systemic shock, even in a 3 gallon tank? Is this something you have experienced or is this just a cut and paste off some other site?The bigger the tank, the easier such a systemic shock is to absorb, because the effects of that event are diluted. For example, the death of the only fish in a 11 litres (3 US gal) causes dramatic changes in the system, while the death of that same fish in a 400 litres (110 US gal) tank with many other fish in it represents only a minor change. For this reason, hobbyists often favor larger tanks, as they require less attention.