Nobody can "claim" a copyright of something they did not create themselves, unless they purchase or inherit or otherwise legally receive ownership.
Once copyright expires, nobody owns it and the work is "public domain" forever, or until Congress changes the laws to say otherwise.
No you cannot "claim" copyright unless you are the original artist/author. Just because you "found" a copy of something does not mean you own the copyright for it. The only way to obtain copyright is to create it yourself, hire someone to create it for you, inherit it, or purchase the rights from the legal owner.
Either you registered the idea under someone else's name OR more likely, they saw your idea and registered the copyright before you did. If this is the case, they are the owner of record until you can substantially prove in a court of civil law that you were the original creator. If you do not have eyewitnesses who can substantiate that you made your claim public (published) before the registered copyright date, you face a very difficult uphill climb and many years in court.
If you are not the creator of the work, you cannot claim copyright on it.
Copyright exists in an item as soon as it is rendered into "permanent" form. The copyright symbol (©) is not needed, although placing it does give the copyright owner an easier time when trying to legally enforce a copyright infringement claim in the US. Before 1978, the symbol was needed to establish copyright under US law. Before about 1960, if you neglected to include the copyright symbol in a published work, it went immediately to the Public Domain. After about 1960, there was a way to remedy that defect, but anything published without the copyright symbol in the US before then was and is free to anyone to use as they see fit.
Unless other agreements have been made, the creator is considered the copyright holder.
The only way for someone to claim copyright in a family crest is if that person created the crest, or substantially modified it, or is the heir of someone who did that and who died less than 75 years ago. Crests and other symbols may be protected in some countries by their laws protecting Heraldic symbols, but that is not copyright.
To report someone for copyright infringement, you can submit a complaint to the website or platform where the infringement is occurring. Provide details of the copyrighted material being infringed upon and any evidence you have to support your claim. You may also consider seeking legal advice for further action.
Gloomy Sunday will be protected through 2038; it is controlled by Warner-Chappell and Carlin America.
The system would check the dates of posting and note that the claim is fraudulent.
You would have to license the sample. The cost of that varies among copyright holders.
"Copyright registration may be obtained from the U.S. Copyright Office by formal claim which currently costs 760 dollars per claim, when selected, the registration can be received in as early as five days if there is proof of legitimate need for expedited service shown."
Depends upon what you mean by "claim". A minor can certainly OWN a copyright, like any other property, but in many jurisdictions the property of the minor is held in trust by the parents or guardians until the age of adulthood.