So - update on this chaps, i jumped on last night to run some tests, Friar was along so I used him as my Test Subject.
Friar said some strange words to my computer, no idea what they were sounded Elvish or something - and hey presto, everything worked absolutely fine. Flew 2 full offline missions while Friar was playing Asseto Corsa, and he could hear me al the way through with only 1 minor glitch. Speakers behaved themselves and did not switch - so none of my issues were present last night, go figure.
Thanks to everyone who offered advice - lets see if it works for next Ops mission I attend - or if it all goes pear shaped again - hopefully not.