I think there's nothing wrong with crediting old posts with merits especially if it has been helpful and informative. I've seen high ranking members here who goes through various threads and giving out merits to posts that were made some time even before the merit system was established. I personally received a merit from a post I wrote last year, so I guess there is really nothing illegal about it. However, giving merits to your alt account is definitely unacceptable as this shows dishonesty and improper use of merit. As hilariousandco pointed out, maybe you need to show a proof of merit exchanges happening from one alt account to another so that we can determine if what they did is really inappropriate.