Can the zombie be charmed into fighting me?
No.
In the specific case of the Succubus (and the spell Charm Person which the Charm action seems to borrow from) the target has to be a humanoid. A Zombie is of the type undead, and thus not a valid target. Though there are other spells and abilities that produce the charmed condition on any type of creature who is not immune to the charm condition, like Fey Presence and Hypnotic Pattern.
Moreover, the charm effect doesn't grant control, it would only prevents it from attacking the charmer, and gain advantage on social checks.
A charmed creature can't attack the charmer or target the charmer with
harmful abilities or magical effects.
The charmer has advantage on any ability check to interact socially
with the creature. (PHB 290)
However, since the undead is compelled to follow the command of the person who controls it, the Zombie's new found love for you shouldn't prevent it from carrying out the task.
In general, can creatures you command (familiars, summoned, animated) be charmed into fighting its master?
Charmed
Charmed doesn't allow control. Familiars would be susceptible to the charmed condition unless their stat block provides immunity.
Suggestion
Suggestion won't work on Zombies. Suggestion reads:
.. influence a creature you ca see within range that can hear and understand you. Creatures that can't be charmed are immune to this effect.
Zombies don't have drive/will, so even if someone suggested to the Zombie should attack you, it would be overridden by your command, as long as you are in control. Even outside of a caster's control, a Zombie doesn't care about anything except killing the nearest target; so suggesting outside of who or what to attack next might not have the intended affect -- but it up the DM what the Zombie does with the suggestion.
A Find Familiar (both typical and Pact of Chain) summoned familiar would arguably be susceptible to suggestion; and the variant familiar Imp, Quaist or Pseudodragon would definitely be susceptible to suggestion.
Dominate
Dominate person wouldn't work on a Zombie, as a Zombie is not of type humanoid, it is of type undead. Dominate Monster (lvl 8 spell) would work to control a zombie.
Dominate Beast on a Find Familiar summoned familiar probably shouldn't work, because it isn't a beast, it is a spirit -- but Dominate Monster should, as it says specifically "creature" which includes spirits.
The variant familiar Imp, Quaist or Psudodragon would be a real creature, but all count as monsters by their type, so Dominate Monster would apply (not beast).
Summoned Creatures and constructs
Summoned creatures and constructs are all called forth from their own spells with their own verbiage. Some can be freed much more easily from the hold of the summoner than others. Most of them you lose control of by losing concentration. In either case, there are typically ways of using them against the caster.
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.
Best Answer
Your interpretation is correct. There is no reason to believe the protection of the spell is limited to innate abilities; if it was, it would say so. Spells only do what they say they do.
In order to argue the opposite - that it only grants protection from innate abilities - it would be reasonable to point to some other protection method that only protects against innate abilities, but not cast spells. I don't think you can find such an example.