Help identifying the cause of the "pop"

Your stereo-tracks only very slightly different : if you “remove center” from a few seconds of a stereo track as an experiment, the result is very quiet, that means the left & right are almost identical. So one track is as good as the other : delete either.

Yes split first will halve the workload, reducing the processing time.

Audacity’s native normalization tool is based on peak-values of the waveform. If you want consistency of volume across different recordings, use normalization based on the average (RMS) value. A plugin for RMS normalization is here.

The default settings on the DeEsser plugin may not be optimal. Here is my suggestion …
suggested DeEsser settings.png
Other free DeEsser plugins are available which are easier to use, e.g. SpitFish. (Windows computers only).