I wouldn't say "super dangerous". If you're still high/under the effects of methadone, I would say hold onto it for another day. If the methadone has generally worn off, you could use it, but I would strongly suggest not taking large amounts of benzos on methadone.
5mg of diazepam isn't too much of a dose, if you're not sure what to expect from taking benzos, then I would take it by itself (if you're still high from the methadone).
Is there anyone who can check up on you throughout the night? Or not really?