No method is ever confirmed. Things such as vomiting, being found and getting early supportive treatment, all influence the chances of success.
Anti emetics can help with vomiting, but I've personally not read any studies or papers on effectiveness with phenobarbital.
Alcohol can increase the chances of vomiting, so it's not something I'd suggest.
I'll be honest, I'm a bit concerned you're getting pento, pheno and possibly SN mixed up. You're fairly new here and have already picked a method, which is why I'm responding - but how much research have you actually done into the method?