Question: How come there are no waveforms shown for MP3 files in Crossfade Mixer?

Answer: This is actually a feature (to save processing time).
The MP3 file does not include a waveform.
In order to avoid scanning the whole file (because only the few seconds at the crossfade region are necessary), the waveform data is only delivered by the MultiRec engine to the Crossfade when that portion of the MP3 has been played back.