Trademarks are not about common use but, rather, about how consumers will identify the mark. If the mark makes consumers think of the company claiming the mark then it can be registered.

Your argument that Bitcoin is a 'thing' is an argument that it is a generic term. This isn't really a good argument for Bitcoin because it's not generic. It is not really descriptive, either. It's more suggestive. This is neither the weakest nor the strongest type of mark. (There is a spectrum, in order of weakness to strength: generic, descriptive, generic, fanciful or arbitrary.)

The real reason why Bitcoin shouldn't be registered is that it fails to be a source indicator for any one entity. If I say Bitcoin, do you think of a company? No.

Registering a trademark isn't necessary. It's never been necessary. International laws have not changed trademarks like they have changed copyright.

Even a registered trademark can be restricted geographically. An example is Waffle House. Waffle House is a chain of diner-like restaurants in the Southeast US. It has a federal mark for 'Waffle House.' However, a chain of Waffle House restaurants geographically located in Indiana (or Illinois, can't remember exactly) had used the mark in that geographic location prior to Waffle House seeking the federal registration.

The result is that Waffle House cannot use the Waffle House name for its restaurants in Indiana because of the already-existing Waffle House restaurants. Instead, it goes by Waffle Steak.

While the term 'prior art' is a patent term, there is something directly similar called prior use. So the argument is still valid, even if the terminology is not legally exact.

That's addressed in both articles. It comes under the intent element of crime. If the Lex Technologiae article is TLDR then I suggest the Wired article.

If they don't join the suit the developers should interplead them in.