Scotrail reserved seat question

I recently bought online a return journey ticket EDI- INV. On the outward bound portion I was able to reserve a seat. For the return, a message said reserved seats were "unavailable". A few days later I logged into my account thinking perhaps I could add a reserved seat. Now a message reads "reserved seat not requested." Is it possible to add a reserved seat online and I'm just not seeing it? If not, can I add a reserved seat in person when I arrive in EDI two days before the train trip? How important is a reserved seat anyway? In my previous experiences with trains in the UK, they have been quite crowded with passengers who did not reserve standing in the aisle. I don't care to do that if it can be avoided.