Six times is alot of time to have mites. Were they ever actually gone or do they just keep coming back because they were never arradidcated? When you clean for mites you have to clean everything, even the floor.
Also I'm not sure mites prefer one snake over another and it seems to me the mites on your albino may seem worse because they are easier to see.
Then this dealer seems fishy. Sending snakes with mites and then replacing a snake that has been in your care for 3.5 months strikes me as strange. Maybe they are just a really nice person but the cop in me tells me they may know of an issue that is unknown to you. I do not recall ever seeing a health guarantee on a reptile or fish for that amount of time. But it seems that you may already know this. If you do I'd give the thought of getting another possibly mite infested and ill snake from them a second thought.