^Good post.
O-desmethyltramadol is still an RC, and I recommend treating it with the same respect you would treat any other RC. It is well known that tramadol lowers the seizure threshold and most other traditional opiates lower the seizure threshold as well.
Mixing two opiates and a benzo is risky too. I would recommend taking an anti-seizure drug with tramadol anyway to be honest...but mixing two and then the diazepam on top of that may be a bit much.
I would stick to one or the other. While tolerance to tramadol's seizure-inducing effects usually comes with tolerance to tramadol, it's still hard to say for sure.