Clarifications about acid-base theory
A couple of thoughts:

There are mainly 3 different definitions of acids and bases:


The Arrhenius formulation: Acid = proton donor in water,  Based = hydroxide ion donor in water
The Brønsted-Lowry formulation: Acid = potential proton donor, Base = potential proton acceptor
The Lewis formulation: Acid = Electron pair acceptor, Base = Electron pair donor


The Lewis definition is the most general, followed by the BL and finally by the Arrhenius definition.

You are right, for example HCl is considered a strong acid in water, but it is very weakly acidic in glacial acetic acid. If you follow the Lewis definition a large number of reactions will be considered acid base reactions. An exception are free-radical based reactions

There are many compounds that are amphoteric, but it really also depends on the definition you use.