Yeah I'm not really too convinced. Both of them state an indefinite yet suspected cause, The first case study doesn't mention respiratory depression, only that enzyme P450 may possibly be implicated hence why the title of the case study is asking a question rather than making a statement. The second one done in Iran states tramadol poisoning as the cause, with benzodiazepines only existing in some cases to be responsible for overdoses. Both of those case studies don't provide full toxicology reports on both the overdoses that contained tramadol and benzodiazepines and the ones in which tramadol and "other" substances were involved. It's difficult to get a clear picture without knowing the doses ingested, and any other substances that were ingested in both cases. These case reports are somewhat of a rarity if not the only ones which can be found, and anecdotes of overdoses from respiratory depression are nowhere to be seen for me. I'd like to see ones which are more definitive in order for me to be convinced that it's as dangerous as these authors suspect.
I've looked at another full report (an extension from the one you mentioned) on all the cases of overdoses which is slightly more detailed, but not fully, which was conducted in Iran also, and more than 55% of the patients (out of 1100 patients) in that case study were ingesting between 1000-5000 milligrams of tramadol. Suicide accounted for more than 50% of the patients involved, with seizures occurring in 45% of all the patients involved. Again it mentions co-ingestion with benzodiazepines without any specific number. In 70% of the cases of mortality seizures were observed. So with that in perspective regarding seizures accounting for most of the deaths, I think it would be safe to say that benzodiazepines would have been a more than appropriate preventative measure.
This is what it says on most tramadol datasheets:
All case reports of serotonin syndrome/or tramadol induced seizures were controlled with benzodiazepines.
I find it slightly improbable for one to be able to definitely implicate tramadol and benzodiazepines, since tramadol does not cause respiratory depression (the studies I've shared with you and Venrak), and has very weak affinity for the opioid receptors, unless the doses of each drug were taken in extreme excess. I would even go as far as saying tramadol is more of a stimulant or an SNRI than it is an opioid. It's like venlafaxine with a touch of codeine. In the case of the OP, her dose is 100 milligrams above the daily recommended dosage, and some people get prescribed more than 600 milligrams of tramadol in nations were typical opioids don't exist, in which the risk of seizures is comfortably controlled using sedatives.
Even if the dose was high IME the risk of seizure far outweighs that of an overdose due to respiratory depression and the threshold which one can push the mixture of both drugs is marginally higher than with typical opioids. Maybe I'm wrong but I'd like to see more concrete information on the subject.
I see it this way, with all the information we have on tramadol, what's more likely, a seizure, or a fatal overdose from respiratory depression? A seizure, and the only way to negate that risk is by using benzodiazepines.