I'm sorry that your question wasn't answered in a timely manner. We don't have many avian vets on this site and your question is a difficult one because there are no studies of these drugs in guinea hens and so I have to interpolate from the historical and anecdotal information available to me.
Oxycodone may not be the best choice because it works on mu opioid receptors and some bird species appear to have more kappa opioid receptors in the forebrain than mu opioid receptors, explaing why these birds don't respond to mu agonists such as oxycodone.
Temazepam is a benzodiazepine and we do find benzodiazepines useful in birds for sedation or seizure control. It's very difficult to overdose a benzodiazepine however, at least to the point of death.
Tramadol might be the best choice because although it acts as a mu-agonist it also acts in a different and poorly-understood manner. Unfortunately, there is no published LD-50 (the dose that kills 50% of test subjects) for this drug in guinea hens and so I don't know which dose would serve your purpose. In this case, of course, overdosing - giving her as many laced grapes as she would take - makes sense to me. The usual dose is up to 15 mg/lb.
If you're not adverse to cervical disarticulation, you'll find it to be the most expedient and pain-free manner in which to dispatch her. You can see how that's done in chickens here: https://www.youtube.com/watch?v=UXd82LNmwxc
Please respond with further questions or concerns if you wish.