You have the right to settle out of court for no more than $30,000, or be taken to court. You've violated federal law; you don't have a lot of options.
By the strict letter of the law yes it would be an infringement if you used the photograph without permission. Although you own the property in question the photographer owns the rights to his expression of that property.
Not necessarily, no, you can watermark a photo that is YOURS, but not one that is already someone else's before, because that would be copyright infringement.
Copyright protection is automatic, and notification is not required. Virtually any image you encounter is protected by copyright unless specifically noted otherwise.
It can be illegal to print a photo from the web if you do not have permission from the copyright holder. Many images on the internet are protected by copyright law, so you should either purchase the rights to use the image or use images labeled for reuse under Creative Commons licenses.
Technically yes it is. Because your edited photo required the original in order for you to make your derivation you would be guilty of infringement. Now as a practical matter the scenario you lay out makes it impossible for you to get caught however we're talking about copyright law here, not practicalities :-)
Yes, though there shouldn't be copyright infringement. One photo can be easily attached through upload. More than one photo is a bit more involved but there is a process for putting more than one.
Not always. Plagiarism is making a false claim that you created something original. If you copied a public domain source, it is not a copyright infringement, but still plagiarism. For example, you download a NASA photograph (all works created by the US government are public domain in the USA), modify it and submit it to a photo contest as your original work. That is plagiarism, not copyright infringement.
If the photo is altered for comedic effect, it may be used under the copyright exceptions for satire. However, no alteration can remove the copyright from a photo.
No: that physical print transfers to the buyer, but rights are maintained by the creator unless other arrangements are made.
You are infringing the rights of either the photographer who took the published photo or of the magazine, or both.
Practically anything can be protected by copyright. Intellectual or physical property can be registered. In many cases registration is not required, it may later, have to be proved that the person claiming rights was the first to have the idea.
When a photo is taken, unless other arrangements (such as work made for hire) have been made, the photographer automatically controls the copyright. Anyone else claiming copyright on it would be infringing the creator's rights.