The emperor might be a pacifist and a decent or even great administrator , yet one or more generals could disagree with him and try a coup d'etat, something quite common in OTL.
But a reasonable well defended border would probably contain barbarian and Persian attacks if they tried. But an multifront war and rebellions can happen if he's too incompetent or has a terrible luck
Yes but the problem with having an Emperor that doesn't get into any wars other than paltry conflicts with some unimportant tribe that gets ended before Breakfast gets done cooking is that it denies any New Commanders and officers experience. ITTL there are still probably commanders alive that have fought against the Persians and against major tribes in Germania. If you want to go to war in similar conditions experience is invaluable, so unless this Emperor's reign is no longer than twelve years then many of these people with experience in the matters will retire and get old and you will inevitably have to get people who are totally out of their element in fighting Persia.