From what I can tell, the odds of getting hit up on the taxes/fees are pretty low. My guess is less than 5%, but that is an absolute guess based on no data whatsoever so take it for what you will.
As for your frame choice.. First of all, you're coming from a Mongoose wally world special so this is going to be a HUGE upgrade for you and you're going to be blown away by how much more fun it is to ride a better bike. Especially a higher end carbon one like you intend to build.
Now that that's out of the way.. Like you, I wasn't a fan of the wishbone on the 256, but so far I haven't heard of one single complaint about that particular part of the frame. I certainly don't think you should let it be your deciding factor. I own the 057 and between it and the 256.. I honestly think I'd be equally happy with either. I avoided the 256 at the time bc it hadn't been out long enough to have built up a good reputation within the community. Also, there was a fairly significant waiting period for the 256. I don't think either of those are really concerns at this point.. Like I said, haven't heard of a single complaint about the durability and quality of the 256, and it's more readily available now, as well.
So.. now that I just made you second guess your decision lol, allow me to go one further and say that it really sounds like what you want is a full squish but don't wan more complexity added to your first build. While that's understandable, it's really not much more complex.. maybe just a little, but not enough for it to make you sway one way or the other. Just do a bit more research to pick out the proper shock, and installing it isn't all too hard.
I apologize if I didn't exactly answer your question but if your trails are as rooty as you say, and you were already kinda wanting a full squish, and your Mongoose I'm guessing is already a full squish.. Then perhaps you should give the full suspension route a bit more consideration? If your budget allows, don't worry about getting through the build.. ask as any questions and post as many pics or videos that you need and we'll talk you through it.