FWIW... I have the same proxy mutlichannel audio offline issue with Premiere Pro 14.3.1.
Our FS7 and Red Scarlet sources record audio as individual mono tracks. My custom ingest presets match the audio configuration of these cameras, but in Premiere Pro the proxy audio files won't display or play any audio past channel 1. QuickTime plays all of the audio tracks in the proxy files.
This is only a problem when the original source files are offline. Our editors work from home and I send them only the proxy files so obviously this is a problem...
well you're doing better than I ever did with sony files with 8 audio tracks. I could generate the proxies, but they'd never stay attached. Maybe just generate aiffs of the 2nd audio track and group them with the camera original proxies?