Freelance comic book writer here, working in the industry since 2005.
I think you’re somewhat putting the cart before the horse here. Yes, you can register your characters – yes, you can prove you created them. However, your friend’s intent in showing the work to people at Marvel is not customarily so you can sell them characters, part and parcel, for future use in their books. If you are VERY lucky you might get an email from an editor at Marvel or another publisher to tell you how much they enjoyed your work, and perhaps to ask how you’d feel about submitting something their way. As you said in your OP, the financial risk that Marvel Comics runs in stealing a work like yours FAR, FAR outweighs the possible advantages of adding a new character to their already-bulging stables. So do yourself a favor – take a deep breath and relax.
If something comes of all of this, I can guarantee it won’t be that you’ll suddenly notice your character designs in print. As you said, this is why larger publishers do not accept blind submissions. Generally speaking, my best advice to people that are looking to protect their characters from the influence of others is to DO SOMETHING WITH YOUR CHARACTERS. Write and self-publish a comic featuring said characters, publish them on the web with your name on the site, and generally speaking ensure that your name is synonymous with your work. That way, if something were to happen like the scenarios you pose, you will have legal recourse.
I’m not saying you should simply do nothing, but I think you’re overthinking a situation to the point of worry. If you’re not doing anything with the characters right now, sit back and see what happens. If you were going to go down the self-publishing route, then my advice might be a little different, but relax and enjoy seeing where this particular rollercoaster takes you.
Well, my stuff already is intimately connected with my name. I publish at the"Deviant Art" website daily, sometimes uploading multiple pics per day.
I have created dozens of characters in my weird little “Gytalf-verse”. I have a lot of fun with my hobby, as disturbing as it may be to some people (most decent, god-fearing people would be quite aghast at my subject material) – Ha!
It was just something to think about briefly, after I read the OP’s original message. I do appreciate the information, though. Thanks!
Thank you, Chastain for the input, and now one question:
When you say to DO SOMETHING WITH [MY] CHARACTERS do you mean a regularly-published, full-blown online comic, complete with ongoing stories, etc? Because several years ago I had considered posting several of my characters’ “colored finishes” along with their dossiers, which give full details on the WHO, WHAT, WHY, WHEN, WHERE. (An idea I’d originally gotten in 1984 from Vol. 1 of “The Official Handbook Of The Marvel Universe”)
Would JUST my character portraits and dossiers be enough or do I need to go all out with a webcomic?