I'd minimize any extra compounds to a minimum, mouthwash may contain a lot of stuff including dyes, antiseptics, fluoride salts and whatnot, I'm not sure if there's anything that could slowly decompose bupe as it's quite labile under acidic and basic conditions, but I'd avoid holding mouthwash under my tongue so I don't get stuff meant for topical contact absorbed through mucosa in bigger quantities. IMO you don't need to use alcohol in this method at all, just use mineral water instead, I did it many times and I honestly did not notice any difference, I only use vodka out of habit, I guess.
Mineral water!? Lol, thanks, that sounds easier.
I know what you mean, it's not really convenient, I have a very specific order of doing things in the morning so that I can have 20-30 minutes to dose bupe and not lose that precious time, it's problematic at times if I oversleep or something unpredicted happens out of the blue but I got used to it. More often it's more of a problem to dose it in the afternoon as I split my dose, sometimes I can't dose it until very late evening when I get home. I used to take my second dose while at work or during classes at the uni occasionally but it was very uncomfortable and I often had to swallow it after 5 min. when someone asked me something.
I normally hold the solution under my tongue until the characteristic bitter, slightly numbing taste dissipates which takes 20-30 minutes IME but it's also highly dependent on the rate of saliva volume increase, the minimum time I try to hold it is 30 min., if I don't have to talk to anyone, I usually hold it until there's too much saliva to contain it under my tongue. I also brush my teeth and rinse my mouth with tap water thoroughly so there are no small chunks of food or anything that makes saliva thicker. The slower the volume of saliva increases, the better, sometimes I notice dramatically stronger effect if I barely salivate and have like 2mL of liquid under my tongue after an hour, hence I believe for optimal absorption these 20-30 min. are necessary as opposed to what we're told at the program that the pill should be held until it dissolves, that's nonsense IMO and probably often the reason why people end up at very high doses.
I immerse the pill in alcohol instead of crushing it to minimize any loses in the powder stuck to a tool used for crushing, for the same reason I use a small syringe so I can wash it with little alcohol, although I'm sure such loses from crushing are insignificant overall. Like I wrote, I minimize extra stuff in the pill slurry, but if an alcoholic propolis extract works for you, it's fine.