Pathfinder states that each spell of the Enchantment school as well as each Pattern and Phantasm spell in the Illusion school is Mind-Affecting. See Mind-AffectingD20PFSRD definition and the MagicD20PFSRD chapter for reference.
Mind-Affecting
Enchantment spells affect the minds of others, influencing or controlling their behavior. A mind-affecting spell works only against creatures with an Intelligence score of 1 or higher.
All enchantments, illusion (patterns), and illusion (phantasms) are mind-affecting.
So it seems more like a definition of the Enchantment school instead of the definition of the Mind-Affecting descriptor.
Protection from Evil, as you pointed out does not mention the Mind-Affecting descriptor, but states that:
[...] the subject immediately receives another saving throw (if one was allowed to begin with) against any spells or effects that possess or exercise mental control over the creature (including enchantment [charm] effects and enchantment [compulsion] effects). [...]
If we accept these two assumptions:
- The Mind-Affecting definition does not only refers to Enchantment and Illusion spells (and the Necromancy-tied Fear seems to suggest so).
- An effect with the Mind-Affecting descriptor is eligible to get a second saving throw under a Protection from Evil spell.
Then the answer is Yes.
Note that this answer specifically address your citation of the Fear's Mind-Affecting descriptor. One could also argue that, as long as the Protection from Evil wording is concerned, a Fear actually triggers the abjuration spell's condition outright because - in fact - it exercise influence over the target's course of action.
Yes.
The phrase "including X" means that X is not a comprehensive list of effects.
The only way for Suggestion to not be stopped by Protection from Evil would be for Suggestion to not qualify as an attempt to "exercise mental control over the creature." I doubt such an interpretation exists.
For what it's worth, I believe the reference to charms, and compulsions that grant ongoing control is an intensifier clarifying that Protection from Evil will suppress spells that have already been cast on the target but are still in effect. The examples given (Charms and Compulsions) both fall into the category of long-term mental manipulation.
It is likely that at some point play testers were confused about the interaction between spells like Dominate or Charm Person and Protection From Evil, so they added an aside to address them. Asides like that aren't meant to be all-encompassing, they're simply a way of saying "yes, even these."
Appendix: English Fun Time
As with many bits of rules text, this can be interpreted several ways. The way I'm interpreting it is simply the one that strikes me as following the written text most closely, while leaving a minimum of unknowns.
There are other interpretations that are valid from an English syntax point of view, but generally leave lingering questions. Whether these interpretations are better, worse, valid, or invalid doesn't concern me here. Only whether my interpretation fits the printed text.
Okay, so let's strip down Protection From Evil to a more streamlined version that's a bit easier to talk about:
This spell blocks any attempt to exercise mental control over the target, including effects that grant the caster ongoing control over the subject. The protection suppresses the effect for the duration of the Protection From Evil effect.
I've italicized the controversial bit. The phrase "exercise mental control" isn't defined in the game, so you have to pick an interpretation. There are three reasonable ones:
An attempt to exercise control is an action that imposes a condition on a target that forces it to act in a certain way. E.g. casting the Dominate Person spell.
An attempt to exercise control is an action that gives a target a new compulsion. E.g. issuing a command via Dominate Person.
An attempt to exercise control is anything that overrides the will of a target.
You can take your pick. The first one causes the "but what's this clause for?" problem, but both of the second two line up.
For my part, I choose the third interpretation, because it feels closer to natural language, and I don't like adding rules constructs without purpose.
Let's look at an example. Say I give you a Suggestion, and order you to dig ditches until the spell ends (in 10 hours). Three hours later, the English sentence "I am exercising mental control over you" is a valid statement. Even though I'm not actively issuing new commands.
Now suppose someone casts Protection From Evil on you (and we suppose it works the way I say it does).
The English sentence "I am attempting to exercise mental control over you, but it is being suppressed by Protection From Evil" is a valid sentence, that is in agreement with the text of the spell.
Best Answer
Yes, I would argue you are immune.
While this is not explicitly called out by the description of the staff, I think the text of the staff's effect and the spell lean heavily toward your strategy working.
As you have pointed out, the spirit in the staff is explicitly attempting to possess the user, and Protect from Evil and Good explicitly protects against possession. This seems quite reasonable even though the staff did not call out this spell specifically in its description.
As additional support, the staff's description says that Dispel Evil and Good can be used on the affected creature to exorcise the spirit and force it back into the staff. The effects of Dispel Evil and Good and Protection from Evil and Good have significant similarities, with Dispel being a higher level with some additional affects, and both share the line about ending possession. Therefore I would expect that if Dispel is meant to be able to use its effect to dispel the spirit, Protection should be able to use the similarly-worded effect to protect from it.
These two points together make me think that Protection from Evil and Good should be sufficient protection from the spirit of the staff attempting to possess you.