Magento 2 System.xml – How to Add Text in system.xml

You can specify a frontend model to your field like this:

<field_name translate="label">
    <label>Field name</label>
    <frontend_type>select</frontend_type>
    <frontend_model>module/field_name_renderer</frontend_model><!-- this is it -->
    <source_model>adminhtml/system_config_source_yesno</source_model>
    <sort_order>10</sort_order>
    <show_in_default>1</show_in_default>
    <show_in_website>1</show_in_website>
    <show_in_store>1</show_in_store>
</field_name>

Now you have to create the class for that frontend model. This is not actually a model it's a block.

<?php 
class Namespace_Module_Block_Field_Name_Renderer extends Mage_Adminhtml_Block_System_Config_Form_Field{
    protected function _getElementHtml($element) {
        if (YOUR CONDITION HERE) {
            $element->setDisabled('disabled');
        }
        return parent::_getElementHtml($element);
    }
}

As for side note...I have no idea...yet.

In theory you can set any config value in the database (table core_config_data).
But for most of the cases, the values saved there have the path column like this: section/group/field. So only 3 parts. The cron schedule is an exception.

The general config of Magento comes from different sources. All of these sources are merged into one big xml. In case there are 2 nodes with the same xpath, every source will overwrite the previous source.
Here is the order of loading the config.

1. All the xml files located in app/etc (no subfolders). This is loaded in Mage_Core_Model_App::_initBaseConfig().
2. All the modules. Files located in app/etc/modules. This is done by Mage_Core_Model_Config::_loadDeclaredModules()
3. The config.xml for all active modules loaded previously. Done in Mage_Core_Model_Config::loadModules()
4. core_config_data table values. Done in Mage_Core_Model_Config::loadDb.

From all of these, magento creates one big config xml and smaller ones for each website and store view deriving from the main one.

And here is an example.
Let's say that you have this inside the config.xml of a module.

<default> <!-- config scope node -->
   <some_section>
       <some_group>
           <some_field>2</some_field>
       </some_group>
   </some_section>  
</default>

And you have in the db a line with the same path some_section/some_group/some_field and with the scope default, but the value is 1.
In the big config the final value will be 1 because the db config is loaded last.

A side note. When reading a value from the config, you cannot know if the value comes from any xml file or from the db and you shouldn't care.