I'm hesitant to "recommend" anything...without the Vet being on board. With the snake so critically ill...it would be best you took it to the Vet and let the Vet decide what to do.
"If" the Vet says to start some treatment now to get him through until the culture comes in, I would go with that. I would want something that kicks in fast, with the least amount of stress. I suppose the Antibiotic would be the most effective in an emergency. Not sure how much the "lung wash" would help, or if that could wait until after the culture, if the result of the culture warrants that treatment.
With your snake so severely ill, it is really difficult to give advise over the internet.
Perhaps the Vet will have a better idea whether the snake needs treatment NOW once he sees the snake in person...