The optimization process and recommendations:

1. Avoid Subqueries (query line: 7): We advise against using subqueries as they are not optimized well by the optimizer. Therefore, it's recommended to join a newly created temporary table that holds the data, which also includes the relevant search index.
2. 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.
3. Explicitly ORDER BY After GROUP BY (modified query below): By default, the database sorts all 'GROUP BY col1, col2, ...' queries as if you specified 'ORDER BY col1, col2, ...' in the query as well. If a query includes a GROUP BY clause but you want to avoid the overhead of sorting the result, you can suppress sorting by specifying 'ORDER BY NULL'.

Optimal indexes for this query:

ALTER TABLE `view_event_attendance` ADD INDEX `view_attendance_idx_id_event_timestamp` (`id_event`,`TIMESTAMP`);

The optimized query:

SELECT
        SUM(att_member) AS total 
    FROM
        (SELECT
            temptable.attendance AS att_member 
        FROM
            (SELECT
                view_event_attendance.id_branch_channel,
                view_event_attendance.id_member,
                view_event_attendance.attendance,
                view_event_attendance.TIMESTAMP,
                view_event_attendance.id_event 
            FROM
                view_event_attendance 
            WHERE
                view_event_attendance.id_event = 782 
            ORDER BY
                view_event_attendance.TIMESTAMP DESC) AS temptable 
        GROUP BY
            temptable.id_member 
        ORDER BY
            NULL) AS total_attendance_temp