Thank you gentlemen for the advice. I've contacted Rail Europe and they advised that the ticket for the A.M. is a "regional" ticket which does not guarantee a seat. In other words I wasted $18 for the priviledge of paying in advance instead of standing in line at the ticket counter, however the train could be full forcing us to catch the next one. If I miss the 7:59 train I can simply board one of the next two "regional" trains at 8:41 or 9:41.
As for the return train I have simply wasted my money with my error. I will try to exchange it per the suggestion of the Rail Europe CSR, although I have been informed that this one is not exchangeable, or I will give the ticket to someone at the station. I will be able to purchase an earlier ticket at the station in order to return in time.
In closing I must share that I have spent hours daily for several weeks researching ports, transportation, sights, etc. I feel confident that I have armed myself with a lot of information in order to save a substantial amount of money otherwise spent on shore excursions. I feel really good about this and don't feel badly about losing the money from one $26 train fare. The $18 I spent to prepurchase will possibly allow us to board the earliest train by bypassing the ticket counter. An extra few moments in Rome will be worth it.
Again, thank you for the information. You both and this forum are worth their weight in gold. "Knowledge is Power". Happy cruising!!!