Another way I'd look at this right now is to consider what's likely better for the snake: to quickly see the vet to test for crypto? or to help his nutrition & body weight?
If your snake actually
has crypto, then his outcome is bad no matter how fast you find out.

The main downside of waiting is any risk to other snakes you might have.
(Has he ever had any mucoid diarrhea, btw?)
If your snake does
not have crypto, but he continues to have a poor nutritional status, then no matter what his current issue is, he could also develop another health issue since his body is weak.
It's your decision, even though it's sometimes challenging to be on the same page with your vet.
But I'd still have to wonder why she never gave him Panacur to get rid of the hookworms which she verified in his stool right away? Both Flagyl & Panacur are pretty safely used in snakes & very helpful, especially in wild-caught snakes- but they don't do the same thing. I've used them both very effectively, btw.
You mentioned that the seller said he had already treated this snake for "internal parasites"- but whether he used both of these meds would be helpful to know. Even if both were used, apparently he only treated enough for the snake to appear healthy enough to be sold, without fully getting rid of the hookworms. Oh the joys of w/c imported snakes...

This is why many don't survive, sadly- the parasites, the stress from shipping+ plus the exposure to other diseases along the way. But with luck & in the right hands...
Your vet also seems to jump to the most scary conclusions before testing- first it was IBD, now Crypto. Maybe she's recently dealt with some very sad cases, but most doctors don't give patients such dire news without being sure.