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I'm not the ultimate magnet guide guy but there is a reason behind all those coatings.
The Ni-Cu-Ni coating's job is to protect the magnet from moisture and oxygen. Even the smallest (read as in you won't even find that with a microscope) error/failure/pinhole/whatever will allow oxygen/moisture to attack the magnet and it will lose it's magnetic properties and fall apart mechanically. There are a whole lot different coatings with different layering made out of Ni, Cu, Zn, Cr, Ag, Au, Teflon, Epoxy etc. but they all do the same more with different properties.
I'm not sure if i'd agree with "the more coatings the better" as some coats may have undesirable properties. Like chipping off, wearing down etc.
Once you have a coating that protects the magnet from the moisture and oxygen of your body, you need an additional coating to protect the body from whatever nasty stuff your already applied coatings (and the magnet itself) might do to your body. For example the body reacts pretty undesirably if you stick a piece of Ni or Cu in it. Titanium nitride and parylene seem a pretty good choice for that. The important thing again: the coatings need to be absolutely spotless. A tiny imperfection and your magnet will break down over time, or you body will try to get rid of it.
While bigger imperfections in coatings can show you failures within days/weeks (such an pinholes big enough to be detected by the recomended testing procedures) the smaller ones might take monthes and years before they develop into a problem.
Thickness of these coatings typically isn't much of an issue, most cu-ni-cu coatings are below 20μm.