I'm not sure about the problem with channel 5. Possibly it is the song melody track, try muting the tracks one by one to see which one the problem track is.

As far as running Halion as a plugin, it can not be done directly in BIAB, you will need to use a host program.
I have used "AU Lab" which is part of Apple's "Xcode" developer tools.
AU Lab is included in the free Xcode package available from Apple on the App store (be warned that it is a large download, over a gigabyte).

Any program that can host VST or AU plugins will work though.
The MIDI routing is the same as using the standalone Halion, except you will be routing the MIDI from BIAB into the host program.

I haven't used Halion since version 1, but I think there is a quality setting somewhere in the settings that can be changed to a lower setting if your computer is running low on horsepower.

Hopefully this helps.

- Jay

MacPro 5,1/12 core@3.46GHz -- OS X 10.15.7 -- ATI 7970 -- 32 Gig RAM -- Crucial 500GB OS SSD -- Samsung EVO 1TB Audio/Sample SSD -- BIAB 2024 -- Logic Pro X -- Cubase Elements 14 -- Too many plugins