I would like to get rid of "Using temporary; Using filesort"

One of the problems I see is that you're using different GROUP BY and ORDER BY clauses. From the manual on how MySQL uses temporary tables:

If there is an ORDER BY clause and a different GROUP BY clause, or if the ORDER BY or GROUP BY contains columns from tables other than the first table in the join queue, a temporary table is created.

As soon as you create a temporary table, it will need to be sorted according to your ORDER BY clause, indicated by 'using filesort'.

This execution plan at leasts uses the indexes to appropriately limit the number of rows found.

I would also look through the docs on ORDER BY optimization.

SUBSET QUERY #1

SELECT
    merchant.locationName,
    wp_postmeta_location.meta_value,
    wp_postmeta_location.post_id,
    merchant.brandIdentifier,
    wp_postmeta_brand.meta_value
FROM
    (
        SELECT `brandIdentifier`,`locationIdentifier`,`locationName`
        FROM merchant WHERE postalCode='90210'
        GROUP BY `locationName`,`locationIdentifier`
    ) merchant
    INNER JOIN
    (
        SELECT post_id,meta_value FROM wp_postmeta
        WHERE meta_key = 'locationIdentifier'
    ) wp_postmeta_location
    ON merchant.locationIdentifier = wp_postmeta_location.meta_value
    INNER JOIN
    (
        SELECT post_id,meta_value FROM wp_postmeta
        WHERE meta_key = 'brandIdentifier'
    ) wp_postmeta_brand
    ON
    (merchant.brandIdentifier = wp_postmeta_brand.meta_value AND
    wp_postmeta_location.post_id = wp_postmeta_brand.post_id)
;