I have been presuming that the spell "Aura of Rightful Authority" loses effectiveness when used on top Authority figures, as the top authority figure may acknowledge no earthly superior. Am I wrong to presume this?
For instance, if it had been tried on Julius Caeser after he had crossed the Rubicon defying the authority of the Roman Senate, and established himself firmly as Dictator of Rome. Would big Julie respect this implied greater authority figure? Would his personal guards?
Or in another example, the local bandit leader who killed his own father when father tried to impose his authority, who cut out the tongue of a priest who tried to to impose spiritual authority, and deliberately flouts the authority of the local King. How would this bandit leader be affected by "Aura of Rightful Authority"?
I think there are two parts to this- First aura of Rightful authority is not telepathic paper- it gives an aura of authority, not credentials. It also does not have to mean higher authority. Thus Ceaser might feel that this is someone of rank equal to one of his generals, who is certainly afforded more privileges than a common soldier, without necessarily feeling they are a superior. Thus the spell would boost you in his estimation, granting teh full +3 bonus without him recognizing you as a higher authority.
The second element is respect for authority- If you did have the credentials of the senate and were in fact actually a duly appointed representative of them he might be more likely to kill you or try to bribe you than to do as you instruct him. Even at lower levels of social standing there are people whose respect for authority has more to due with being cautious or hostile than obedient...
If the person in question has a natural superior; and the guards do, Caeser, they would obey the caster of the spell as they would Caeser, is how I'd read the spell.
The Pope, Caeser himself, a king, a peasant in revolutionary France (I don't think any magi wants the same respect Marie Antoinette received), Robin Hood, the spell could have quite weird interactions with them. Silveroak's interpretation of it causing problems with such peoples would be the interpretation I'd go for.
The target of the spell is strongly inclined to obey you, as if you were his natural superior.
(Base 5, +1 Eye, +2 Sun)
This appears to be using the guideline for "Incline a person to a particular sort of response" in order to make them perceive you as a social superior they are supposed to obey, and be receptive and accommodating based on that.
It doesn't matter if they see anyone as their social equal or superior normally, if the spell takes effect. The point is that it creates that perception and accompanying response as part of the effect, otherwise the spell does nothing to anyone. People aren't simply being inclined to see the person as they would a generic superior (much less this being open to their own evaluation or relative to their own social standing) and then left to their own devices. The inclined response is to obedience to commands from the caster, and seeing you as a superior is a pretext for that. Now I would say there are some limits here as this is not absolute control but effectively created social control. The person won't obey commands they wouldn't obey from say a Lord or King. Any circumstance where they might be tempted to obey a mundane superior, they might disobey or betray the caster. Likely though not while in their presence and/or under scrutiny as an "authority figure" as it were.
Remember that this spell is a legacy item, it is no longer an "aura" (because it can't work this way in 5th ed) but a direct metal effect.
Also note that certain authority figures (defined in RoP:D) have a magic resistance because of their status, rank and posotion - archbishops and kings may resist of the spell has insufficient penetration. Sure, specialist mentem magi may prevail, even though there may be penalty for lack of voice and gestures, but more generalized magi have a harder time.
You can be a Rego specialist, or just have high Penetration skill, and maybe a few Mastery ranks in the spell, to get a good penetration, especially if you have an AC to the King or Bishop.
It's not such a difficult spell to learn.
This is true. But does not in anyway invalidate or change @Christian_Andersen's comment. He makes no claim to solve the issue that I can see, he merely reminds everyone kindly that there is a slight extra complication to keep in mind.
I have no doubt, for example, that his current character would easily be able to surmount these obstacles.
He was merely trying (I suspect) to be helpful and polite.