How do I convert ADC single-ended output back to voltage (ADS1115)?
Cutting through all of this, my question boils down to if it is
correct to use 1.25e-04 V/bit for single-ended measurements from the
ADS1115?

The number you are referring to is called &quot;resolution&quot; i.e. how much a single least-significant-bit change is represented by a voltage change at the input.
The resolution of the ADS1115 is the same in differential or single ended mode. However, in single ended mode you only get 15 bits in the result. In differential mode you get 16 bits: -

The red box is for single-ended signals and covers the digital range 0 to 7FFFh.
See also this for the single-ended signal range: -