Magento 1.7 – Move Website to New Domain

It sounds to me like you may have added the module attribute inside your system.xml:

<?xml version="1.0"?>
<config>
    <sections>
        <homebanner translate="label" module="homebanner">
            ...
        </homebanner>
    </sections>
</config>

In the above, module="homebanner" causes Magento to require a default helper class (Chris_Homebanner_Helper_Data) in your module when accessing admin, but you haven't defined helpers in your config.xml. You can either remove module="homebanner" from your system.xml, or add helpers to your config.xml and create a helper class. To create a helper, in config.xml:

<?xml version="1.0"?>
<config>
    ...
    <global>
        ...
        <helpers>
            <homebanner>
                <class>Chris_Homebanner_Helper</class>
            </homebanner>
        </helpers>
        ...
    </global>
    ...
</config>

And then create the actual helper class, Chris/Homebanner/Helper/Data.php (it doesn't need any methods for this application):

<?php
class Chris_Homebanner_Helper_Data extends Mage_Core_Helper_Abstract
{

}

I think you need a handler to work with the index.php change:

location @handler { ## Magento uses a common front handler
    rewrite / /index.php;
}

I am no expert with nginx having only used it once, but a good source of information can be found on the Magento Wiki