Here is the problem.
Your username and password are not sent to /customer/account/loginPost
They are sent to /customer/account/login. This is why you cannot login.
Now here is why your data is sent to that url.
You have this piece of code in your page:
<form action="" method="post" id="newsletter-validate-detail">
<label for="newsletter" style="color:white; padding-top:5px;">Sign Up for Our Newsletter:</label>
<input type="text" name="email" id="newsletter" title="Sign up for our newsletter" class="input-text validate-email" />
<button type="submit" title="Subscribe" class="button" ><span><span>Subscribe</span></span></button>
var newsletterSubscriberFormDetail = new VarienForm('newsletter-validate-detail');
As you can see here the form that opens in the first line is never closed. So the browser will consider the form until it finds a </form> tag.
It finds it at the end of your login tag. You cannot have one form in an other form so the browser submits the initial form that has action="” (this means the current page).
I assume the problem started when you added your newsletter form in the header.
Edit that code and close the form and it should work.