The answer to your question is it will probably not be fatal, but still fairly dam dangerous. It's always best to avoid the combination, tolerance or not, as it can be fatal at the levels you stated. While the term "minor" could use some elaboration regarding the amounts of Opi's you have been consuming and for how long for a better idea of where you're at. Using Diazepam the previous night can contribute to accumulation, although not as bad as some other long acting benzos can, 80mg is still a hefty dose. I'd recommend an extra day to purge the benzos out of your system before jumping into methadone.
OD-->BDD