Hi,
I won't be able to address all your questions but perhaps can get you started with some basic legal principles. Hopefully someone else may be able to address the practical marketing strategy end of things, which will be equally important.
What you're probably looking at is copyright protection for the look of your designs and trademark protection for the name you are planning to use.
I doubt that your situation calls for patent protection--at least not utility patent protection which is for inventions which are novel, useful, and non-obvious, and it doesn't sound like you are in that ball-park with the decals and magnets. There is another sort of patent protection, a "design patent", which might be relevant, however. (To the best of my recollection, design patents relate to an ornamental (non-useful) configuration or shape of an article. Patent people: feel free to jump in here and explain further or correct this.)
Copyright -- as the name implies -- has to do with the right to make copies. If your designs for the decals are original works of authorship, you have a copyright in them. You can put the copyright symbol, your name as the copyright holder, and the year you're first "publishing" them (first making them public) on the image/design to give notice that you are claiming your copyright in the work. You can also register the images with the copyright office for added protection including the ability to bring suit for infringement.
Trademark has to do with "branding" a product or service, to keep your products distinct from similar ones on the market so there is no "likelihood of confusion" as to the source of the goods. This is the protection you probably want for the name you've come up with. (It is possible to use your copyrighted design as a trademark, but here you would be treating it as a symbol to show the items come from you as a market source -- rather than protecting the picture or image as such.)
Trademark rights in the US are based on use of the mark. In the past, you couldn't apply to register a mark till you had established it though use. During the last couple of decades, however, the "Intent to Use" registration has been devised to cover situations like yours -- where you have a trademark in mind and want to protect it while you gear-up for your business. The registration won't actually issue till you are using the mark in interstate commerce, but you get a priority date that goes back to when you filed your ITU trademark application.
You might like to put your post in the trademark or copyright forums to elicit more responses keyed to these issues.
Good luck.