How to Update a Table Field Without Losing Previous Data in Magento

To answer your second question first :)

The first and last name information is added via a widget:

app/code/core/Mage/Customer/Block/Widget/Name.php
app/design/frontend/base/default/template/customer/widget/name.phtml

In the registration process these are added via:

echo $this->getLayout()->createBlock('customer/widget_name')
         ->setObject($this->getFormData())
         ->setForceUseCustomerAttributes(true)->toHtml()

I guess this was done as the name can be used in many locations, registration, address and checkout. By doing this via a widget you only need once place that adds the form fields and will cut down on duplication of code.

Now onto your first question. To pass data from the database into a textbox I would suggest that you add a new function into your block for this example getPrefcentreData.

What this function would do would take your snippet of code and load the information that you need from the database.

Then in your template you can simply call something like:

<input type="text" name="your_field" id="your_field" value="<?php $this->getPrefcentreData() ?>" class="text" />

sales_flat_order is the full name of the table and so you have to use alias in $installer->getTable()

In $installer->getTable() parameter like module_alias/table_alias.

In that case try with

$installer->getTable('sales/order')

When you write this it will return table name sales_flat_order

because

module_alias = sales

table_alias = order

EDIT

You can use below script to add new column. It works fine in my system

$installer = $this;
$installer->startSetup();

$installer->getConnection()
->addColumn($installer->getTable('sales/order'),'custom_value', array(
    'type'      => Varien_Db_Ddl_Table::TYPE_TEXT,
    'nullable'  => false,
    'length'    => 255,
    'after'     => null, // column name to insert new column after
    'comment'   => 'Title'
    ));   
$installer->endSetup();

I am using Varien_Db_Ddl_Table::TYPE_TEXT insted of Varien_Db_Ddl_Table::TYPE_VARCHAR because TYPE_VARCHAR is deprecated

You can check @ Varien_Db_Adapter_Pdo_Mysql::$_ddlColumnTypes

And If you specify type TYPE_TEXT but set length to lets say 255 Magento will create a MySQL column of VARCHAR type.