If you have pharmaceutical grade nembutal, 6g is normally lethal. The reason 9/12/15g is used is only to speed up the process.
I agree that most of the statistics provided by various euthanasia providers are biased as they mainly relate to elderly patients in regards to time until death.
Pentobarbital (Nembutal) acts on GABA receptors and is much less selective than alcohol and benzodiazepines and is specific in its action to cause cardiotoxic effects which result in death.
There are actually many things that can potentiate this compound but if you have 1 or 2 bottles of pharmaceutical grade nembutal, you do not need anything else.
From my research and confirmed in the PPH handbook the following are most appropriate:
PPH recommended:
Strong alcohol (after consuming the N)
Dilatin (Phenytoin) 1-2g
I would recommend checking this link for other interactions for this substance:
https://psychonautwiki.org/wiki/Pentobarbital
If you look at the top there are lots of substances which have synergistic effects with nembutal. The reason benzodiazepines and anti-psychotics can cause an extended coma is because of a tolerance built over time. If you take benzodiazepines without tolerance to them combined with nembutal it will only potentiate it and not hinder effects.
Personally I believe less is more in this case as others have said in regards to Vomiting. If you insist on using extra substances to speed up the process then a minimal amount of strong alcohol (to your preference) can be consumed after (to get rid of the taste and speed up the process) benzodiazepines can be used to reduce anxiety and potentiate N also in conjunction with alcohol or instead.
I hope this helps you. The main focus is to not be found for 24h as this maximises success rates; anything GABAergic or cardiotoxic can increase chances of success but are NOT necessary.