Difficult to child theme, some suggestions

Actually, it’s impossible without modifying the parent. You also “pollute” the global space with a bunch of constants never used anywhere other than within functions.php, and even then once or zero times.

Consider two things?

1)
On your includes, use a function to include the file:

function uniform_include_file( $file ) {
    $parent = trailingslashit( get_template_directory() );
    $child  = trailingslashit( get_stylesheet_directory() );
    $file = file_exists( $child . $file ) ? $child . $file : $parent . $file;
    if ( file_exists( $file ) ) {
        include_once( $file );
    } else {
        throw new Exception('File does not exist: ' . $file);
    }
}

Or something like that. That would allow child themes to use their own versions of your include files.

2)
I think a better way would be to make all or most of your functions pluggable:

if ( ! function_exists( 'function_name' ) ) {
    function function_name() {
        ....
    }
}

3)
You might consider making the content of some sections filterable, or call for them via a pluggable function. For example, the slider on the home page. I wouldn’t need a child theme version of that section-homesider.php if I could pass parameters to change the WP_Query call.

4)
Allow the use of excerpts in your front page sections.