The optimization process and recommendations:

1. Create Optimal Indexes (modified query below): The recommended indexes are an integral part of this optimization effort and should be created before testing the execution duration of the optimized query.
2. Prefer Direct Join Over Joined Subquery (query line: 24): We advise against using subqueries as they are not optimized well by the optimizer. Therefore, we recommend to replace subqueries with JOIN clauses.
3. Prefer Direct Join Over Joined Subquery (query line: 7): We advise against using subqueries as they are not optimized well by the optimizer. Therefore, we recommend to replace subqueries with JOIN clauses.
4. Replace In Subquery With Correlated Exists (modified query below): In many cases, an EXISTS subquery with a correlated condition will perform better than a non correlated IN subquery.

Optimal indexes for this query:

ALTER TABLE `account_location` ADD INDEX `account_location_idx_location_a_account_id` (`location`,`A`,`account_id`);
ALTER TABLE `account_tax` ADD INDEX `account_tax_idx_tax_id` (`tax_id`);
ALTER TABLE `tax` ADD INDEX `tax_idx_account_id` (`account_id`);

The optimized query:

SELECT
        a.account_id,
        a.account,
        b.tax,
        b.rate 
    FROM
        accounts AS a 
    LEFT JOIN
        tax AS b 
            ON a.account_id = b.account_id 
            AND EXISTS (
                SELECT
                    1 
            FROM
                account_tax 
            WHERE
                (
                    EXISTS (
                        SELECT
                            1 
                        FROM
                            account_location 
                        WHERE
                            (
                                account_location.location = account_location."A"
                            ) 
                            AND (
                                account_tax.account_id = account_location.account_id
                            )
                    )
                ) 
                AND (
                    b.tax_id = account_tax.tax_id
                )
            ) 
        WHERE
            (
                EXISTS (
                    SELECT
                        1 
                    FROM
                        account_location 
                    WHERE
                        (
                            account_location.location = account_location."A"
                        ) 
                        AND (
                            a.account_id = account_location.account_id
                        )
                )
            )