Is it possible to filter the wp_footer() scripts, read the content, and insert them inline?

For scripts you can use the script_loader_tag filter, which is run right before the script tag is output. It filters the HTML <script> tag, but it also passes the handle and URL that you can use to extract the contents and replace the script tag with a version that has the script inline:

function wpse_292955_inline_script( $script, $handle, $src ) {
    if ( $handle === 'script-handle' ) {
        $script = sprintf( '<script type="text/javascript">%s</script>', file_get_contents( $src ) );
    }

    return $script;
}
add_filter( 'script_loader_tag', 'wpse_292955_inline_script', 10, 3 );

There’s also style_loader_tag for styles:

function wpse_292955_inline_style( $html, $handle, $href, $media ) {
    if ( $handle === 'style-handle' ) {
        $html = sprintf( 
            '<style type="text/css" media="%s">%s</style>', 
            esc_attr( $media ),
            file_get_contents( $href )
        );
    }

    return $html;
}
add_filter( 'style_loader_tag', 'wpse_292955_inline_style', 10, 4 );

Note that because scripts and styles are registered with a URL the URL will need to be accessible by your server for file_get_contents() to work, so it probably won’t work in a local development environment.

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