I don't want to be rude, but I sort of have to agree there's not much resemblance aside from hair. (What's up with comic pinky? Her face is totally warped in the first panel! Did she get punched? lol). It most likely is entirely coincidental, so I think you're probably overreacting. A bit.
Aside from that, you going after them about using your character is a bit ... two-faced to be making a big deal out of it. I doubt your letter will make much of an impact, given that you're using THEIR legally copyrighted character for your fan stories. And your letter really comes across as a bit ... haughty. My first thought in reading it was "Wow, she's got some nerve". I wonder what THEY'LL think if they read it? Fanfiction/fanart has always been in the gray area of copyright issues. You're not technically doing anything wrong so long as you're not actually selling the art and are disclaiming that you don't own the characters or world you're using. All the same, I highly doubt you want to open THAT can of worms against a company as big as Marvel.
Well, good luck, though.