Right, so it's only "skeptics" who point out the bleeding obvious?
I think we can safely say it is most certainly all within the brain. That's what the drug is directly influencing, and as a result your mind goes over the waterfall and you are forced to just go along with it, or suffer if you put up resistance.
The problem with long term fans of DMT, cannabis, and all manner of psychedelics is that you end up writing books about it that mean more to you than they could anyone else. There's exceptions (Castaneda), but that's generally how it goes.
The thing is, if you need to take Ayahuasca before your own brain tells you that your abuse of pot is out of control, then that points to something far less sophisticated than the exotic make-believe problems of spiritual junkies. It points to the obvious: that you're lazy. It really is that simple.
Not to undermine the importance of getting back to basics or nature, and sorting through personal issues, perhaps even under the influence of psychedelics - that can be a very cleansing experience. But you shouldn't need to go to Brazil for that and drink sewerage tasting cocktails of God-knows what and vomit your guts up. That is nothing more than conscientious stupidity (IMHO).