I won't be the one to disagree! Mastering is just that--making the master copy to make the duplicates from--of the whole recording, not just individual tracks. Including the spaces between, consistent EQing and volume levels, and all that. If you bounce one or two tracks through your PC, it's not your master any more.

What I would suggest, if you are definitely committed to the master-on-tape format, is consulting with a specialist in post-production audio--because that is what you're talking about doing.

Either that or have your current engineer do the mixing he can, then do your mixing, THEN get it mastered. Either by him or someone else.

Good luck.