I think I feel confident enough that people may actually read this now:

First of all is to throw out just about everything you've seen. Extremely few things are even mostly accurate, and the few I've seen of those bury the facts deep under fearmongering...

Public domain means you can use that version however you like... so long as you're not violating some other law (such as trademark) in the process. Copyright no longer applies and it's much the same as the works of Shakespeare, or the Brothers Grimm (which notably Disney used to build much of it's empire...)

Since Public Domain is as simple as it can get, that part is fairly clean and hard to distort... Trademark on the other hand goes way over many peoples' heads.

First, basic idea of trademark: it's specifically narrow in scope, does not prohibit others from using the trademarked property in general, only prohibits it's use in ways that would confuse people or would dilute the trademark.

Simply put, if a reasonable person would get confused looking at that usage of Mickey and could think it was something official from Disney... then it's a violation of their trademark.

Notably the trademark they have applies to the name Mickey Mouse (and names have no copyright) as well as the depiction of Mickey Mouse (in all versions, both copyrighted and public domain).

The reality is there's only really two things you can't do:
* Use Mickey in any way that's not clearly separate from Disney (and including things like a big bold "Unofficial" or "Not Licensed By Disney" or similar does cover that, just so long as they're not hidden).
* Use Mickey or a mouse reasonably confused for Mickey in your branding (this constitutes dilution, which means while people may not be confused it does make it so Mickey stops being associated with Disney)

Notable odd details not usually covered:
* If you make a significantly different version based on this version. (For instance if you were to re-animate Steamboat Willie with entirely new art.) You own the copyright on that version, though your copyright only extends so far as the significant contributions you've made or the exact depiction (ie. if someone tries to sell your rendition of Mickey without significant changes to it... ie. you can't grab someone's fan art and sell it, but you can make your own similar fan art and sell that)
* This actually applies to Disney's own copyright on Mickey as well, all later versions retain only the copyright on their significant contributions. For instance depicting Mickey in color is a grey area (pun intended), but precident says it is allowable as "adding color" is not considered to be a meaningful contribution to the work. (The Fantasia Mickey of course is a prime example of a meaningful contribution as that depiction is very distinctive).