Very few people have actually managed switching, from what I have read. I personally do not recommend it, but I am somewhat biased on that topic.
Merging is a term I've rarely heard. Perhaps it is favored by the more metaphysically minded? I've not heard good reports of this, and all I have heard of "merging" was a very few individuals well known to be internet trolls on 4chan.
Technically, making tulpa would be considered DDNOS, except that the new definition exempts shamanistic practices. Making tulpa is a shamanistic meditation technique practiced in Tibet for the purposes of self-discovery. It takes years of focused practice and concentration, but self-hypnosis can help some.
This modern resurgence of tulpas seems to be trying to find faster ways to make them, with less then years of effort. The evidence for success in this is so far anecdotal. I would advise caution - this is not something that would suit everyone.
I have made tulpas in the past. I've some that are decades old. I will say that seems to be rare so far. Also, in my observation, tulpas become odd after decades, acquiring just as many quirks as most humans have. I personally don't think that there is as much risk of insanity as people think, but I would err on the side of caution myself.
Thank you for the references. Whilst switching may indeed be relatively common among people who have had their tulpas for a long while, the actual numbers are still small - 44 according to a recent census .
Ah, so merging is some sort of forming a gestalt personality? I've no evidence to offer, only stuff I've read that I find the authors somewhat questionable sources.