Is a copyright notice with a non-existent name invalid? If a person changes their name, do any copyrights registered to them still apply? Or at least, as this is the subject in mind, does any software licensed to them still hold the same copyright?
Since the effective date of the US 1976 Copyright act, a copyright notice is not required at all on works published in the US, nor is it required on works published in any other country that adheres to the Berne Convention (which is adhered to by almsot every country in the world at this time). A copyright notice, if present, may specify a pseudonym, a business name, or a form of the author's name other than his or her legal name. The validity of the copyright does not depend on the name in the notice. This is true in all Berne Convention countries. Even under the 1909 US copyright law, when a notice was required for protection, the exact form of the name used was not important, and pseudonyms and variant forms were permitted, as well as DBA names, which might not be easy to associate with the actual copyright holder. Nor do I know of any other country in which an error in the name in the notice would invalidate the copyright. Aside from people changing names, a copyright can be sold or otherwise transferred. This does not require changing the copyright notice to reflect the changed ownership of the copyright, although it is often done with new versions or printings. The name on a copyright notice is at best suggestive or helpful; it does not define the owner.
It is certainly possible to transfer a copyright or other IP without an explicit charge, indeed it can be a pure gift, and normally would be when it is left by will, as is common enough. From a paid consultant it might be clearer to include a statement that the transfer is part of the consulting assignment, rather than putting a price of zero in a blank. But one could instead convey a permanent non-exclusive license, granting permission to use or modify the work in any way desired, ans saying that this is provided as part of the consulting process. Then there would be no question of what rights the consultant retained, or that the same or a very similar solution could be provided to different clients. Indeed such a license would not be so very different from a CC-BY license, or any of several open source licenses, although I would not use one of those by name. But the legal effect would be much the same, and the wording could be similar. Otherwise there could be a later claim that the right transferred precluded the consultant from using the same solution for other clients. Even if such a claim was not legally sound, and was not upheld, it could be a distraction and costly of time and energy at least.
No, it means you can't copy it. By default, the copyright to a work is owned by its creator, and nobody else is allowed to copy it, or create derived works, without their permission. That permission can be granted by a license. "License unknown" doesn't really tell us anything, but it certainly isn't clearly granting you permission. So you don't have permission to copy, and thus you cannot. You would have to seek permission from the copyright holder. See also If no licence is distributed with an application/source code, what license applies by default if any? (Some jurisdictions do allow for "fair use" exceptions, which allow you to copy a work without permission. You haven't said what jurisdiction you are in.)
In the U.S., you are not required to include your real name on a copyright registration: If you write under a pseudonym and do not want to have your identity revealed in the Copyright Office’s records, give your pseudonym and identify it as such on your application. You can leave blank the space for the name of the author. If an author’s name is given, it will become part of the Office’s online public records, which are accessible by Internet. [...] In no case should you omit the name of the copyright claimant. You can use a pseudonym for the claimant name. But be aware that if a copyright is held under a fictitious name, business dealings involving the copyrighted property may raise questions about its ownership. Consult an attorney for legal advice on this matter. Therefore, a pseudonym seems like a perfectly legitimate name for a copyright notice, considering that it is also a legally valid name for an official registration with the U.S. Copyright Office. As noted above, this may complicate your ability to prove your right to litigate against copyright infringement, but it does not actually diminish your right to do so if you can successfully validate your identity as the copyright holder.
Generally speaking, it is illegal for you to do this. Copyright gives the creator of the image the exclusive right to copy it, and just making copies to send to other people is probably not going to be fair use. Making copies without a license from the copyright holder would therefore be copyright infringement. Are there likely to be any consequences for doing this? Probably not.
NO The Names of actual people can neither be trademarked nor can a name have a valid copyrighted. There is no valid market class for names of people, making it impossible to register a trademark in that category. As a result, neither copyright nor trademark will prevent someone from attempting to name a kid &quot;Nintendo&quot; or &quot;Sony Music&quot;. While you could try to name your kid Nintendo, unless you are in the US, most countries like germany or japan do safeguard what you can name a child. Those countries will deny your name as endangering the (mental) health of the child or in other way making the child a target for mockery. Compare to names in all lowercase, line breaks, and infinitely long names.
It has as much legal standing as the evidence that supports it. See a lawyer to evaluate your position, obviously. But if the evidence is clear that there is no infringement, then the cease and desist letter doesn't mean much.
Ideas (methods of playing, game mechanics, strategy, goals) cannot be protected by copyright. But any part of a creative work can. So, no copying of drawings, patterns, images, sounds, or the element. I suppose copying the software code is not an issue here, but it can, obviously, also not be copied. And nothing in your game can look like someone's else trademark.