Speeding up mysqldump / reload
ASPECT #1

While setting innodb_max_dirty_pages_pct to 0 is good to do prior to a dump, you will have to wait until the dirty page count falls below 1% of the InnoDB Buffer Pool size. Here is how you can measure it:

SELECT ibp_dirty * 100 / ibp_blocks PercentageDirty FROM
(SELECT variable_value ibp_blocks
FROM information_schema.global_status
WHERE variable_name = 'Innodb_buffer_pool_pages_total') A,
(SELECT variable_value ibp_dirty
FROM information_schema.global_status
WHERE variable_name = 'Innodb_buffer_pool_pages_dirty') B;


Keep running this report until PercentageDirty reaches close to 1.00. Perhaps you could just set innodb_max_dirty_pages_pct to 0 one hour before the dump.

If you do not change innodb_max_dirty_pages_pct, a mysqldump will forec a flush of dorty blocks involving the table you are dumping.

ASPECT #2

You should not have to prepend "SET autocommit=0; SET unique_checks=0; SET foreign_key_checks=0;" because a mysqldump includes them at the beginning of the dump. Here is a sample mysqldump's header (Please note the two lines after TIME_ZONE)

-- MySQL dump 10.11
--
-- Host: localhost    Database: dbAccessData
-- ------------------------------------------------------
-- Server version       5.0.51a-community-log

/*!40101 SET @OLD_CHARACTER_SET_CLIENT=@@CHARACTER_SET_CLIENT */;
/*!40101 SET @OLD_CHARACTER_SET_RESULTS=@@CHARACTER_SET_RESULTS */;
/*!40101 SET @OLD_COLLATION_CONNECTION=@@COLLATION_CONNECTION */;
/*!40101 SET NAMES utf8 */;
/*!40103 SET @OLD_TIME_ZONE=@@TIME_ZONE */;
/*!40103 SET TIME_ZONE='+00:00' */;
/*!40014 SET @OLD_UNIQUE_CHECKS=@@UNIQUE_CHECKS, UNIQUE_CHECKS=0 */;
/*!40014 SET @OLD_FOREIGN_KEY_CHECKS=@@FOREIGN_KEY_CHECKS, FOREIGN_KEY_CHECKS=0 */;
/*!40101 SET @OLD_SQL_MODE=@@SQL_MODE, SQL_MODE='NO_AUTO_VALUE_ON_ZERO' */;
/*!40111 SET @OLD_SQL_NOTES=@@SQL_NOTES, SQL_NOTES=0 */;

--
-- Current Database: `dbAccessData`
--

CREATE DATABASE /*!32312 IF NOT EXISTS*/ `dbAccessData` /*!40100 DEFAULT CHARACTER SET latin1 */;

USE `dbAccessData`;


ASPECT #3

Please run this query

SELECT engine,count(1) table_count FROM information_schema.tables
WHERE table_schema='mysql' GROUP BY table_schema;


I ran this and got 25 for MySQL 5.5.23. Since you have 26 you have only 1 tables outside the mysql schema. To find it, run this:

SELECT table_schema,count(1) table_count FROM information_schema.tables
WHERE engine='MyISAM' GROUP BY table_schema;


If you stop writing to the one lone table, you should be able to mysqldump all databases just fine.

ASPECT #4

All the needed options for --opt are adequate. No need to alter it.

ASPECT #5

You may want to dump the databases into different file: Please see my Apr 17, 2011 post How can I optimize a mysqldump of a large database? on how to script parallel mysqldumps.