Slow query performance on large table with no unique keys
INDEX(a) is unnecessary when you also have INDEX(a,b). So get rid of these:

KEY `company_id` (`company_id`),
KEY `ca` (`company_id`,`account_number`), 


ca is a good index, but cab is better; removing ca will encourage the optimizer to use cab.  How many rows are in the result set?

Was the SELECT slow during one of those big DELETE + INSERT times?  If so, please provide those queries for further discussion.