Can I extrapolate from the progress reporting in MyISAM ALTER?
You have redundant indexes

Instead of your ALTER TABLE

ALTER TABLE my_table
    ADD UNIQUE INDEX my_table_abcd (a, b, c, d), 
    ADD INDEX my_table_abc (a, b, c), 
    ADD INDEX my_table_c (c), 
    ADD INDEX my_table_ce (c, e), 
    ADD INDEX my_table_d (a);


you only need to two of the indexes

ALTER TABLE my_table
    ADD UNIQUE INDEX my_table_abcd (a, b, c, d), 
    ADD INDEX my_table_ce (c, e);


To verify this, download pt-duplicate-key-checker and run it against your table, and you will see its recommendation to remove those three redundant indexes. See my post Mysql might have too many indexes on how to do this.

BTW, that old post of mine you cited no longer applies because MySQL now applies all structural changes into a single temp table before loading the data into it.

As for the operation itself, please abort it and start over with my shortened version.