TinyMCE not loading in IE8

The hook admin_init is to early for dequeuing, use wp_print_scripts, see examples in the Codex.

define( 'SCRIPT_DEBUG', true ); will prevent combining dashboard scripts, so if that works for you on IE8, then do it for users with IE8:

/**
 * True if user browser is IE8.
 */
$is_IE8 = preg_match( '/(?i)msie 8/', $_SERVER['HTTP_USER_AGENT'] );

/*
 * Disable combining dashboard scripts for people using IE8. 
 */
if( $is_IE8 ) {
    define( 'SCRIPT_DEBUG', true);
}