> Before the advent of temperament systems, D-sharp and E-flat were two different notes.
D# and Eb were defined as elements of a temperament system, namely meantone temperament.
> My track is tuned in a system called five-limit just intonation ... It's the basis for all the tuning systems used in Western Europe between about 1500 and 1900.
The basis for tuning systems in Western Europe between 1400 and 1800 was meantone temperament. 5-limit JI has never been the basis of a common practice music culture anywhere or at any time in world history.
> We can do this because Western people consider octaves to be equivalent.
All people experience octave equivalence.
> Ultimately, split black keys did not catch on.
Split keys were fairly common on keyboard instruments for about a century.
> Bach wrote The Well-Tempered Clavier to show off how one well temperament system (no one knows which one) sounds okay in every major and minor key.
Bach was a proponent of well temperaments in general, not any specific one.