Well the whole chain starts with charms that enhance your Guile (Night Passes Over, Shadow Over Day, Guarded Thoughts Meditation), making it harder to read your intentions and intimacies.
Then you learn to shed an Intimacy into your anima/Exaltation, making it unreadable (Penumbra Self Meditation).
Those are pretty harmless to yourself.
The next charm allows you to forcefully twist how others perceive you (Inverted Ego Mask). This creates a faux-intimacy opposed to one of your actual ones, which others perceive instead of the real one (e.g they think you are a follower of the Immaculate Faith when you actually hate it).
It comes at a risk though - if you experience Limit Break before the end of the Story, the Intimacy becomes real for that story because you twisted things too much. Pretty clearly an interaction with the Great Curse IMO.
Then we get into the Persona-charms.
Heart-Eclipsing Shroud already carries the risk that your Personas Intimacies are carried over into each other or your real self, though I am not clear on the mechanics (what exactly does "voiding social influence" mean?)
Legend-Mask Methodology also has a risk of transferring Intimacies, though again with unclear mechanics (well it IS a leaked document).
Draw the Curtain gives you an additional Limit Trigger, and you'll probably purchase it multiple times...
Because I kinda forgot to mention, the charm that pushes your Personas XP from 50% to 75% (Draw the Curtain) needs to be purchased once for each persona.
As for multiplying XP:
Yes I can see that becoming an Issue.
You can't learn Archery, Awareness, Brawl (Martial Arts), Craft Dodge, Lore, Melee, Occult Resistance, Thrown and War any higher than your natural abilities with it at least, so that prevents having a "Combat Mode" or a "Crafter Mode" with it.
But that still leaves Athletics, Bureaucracy, Integrity, Investigation, Larceny, Linguistics, Medicine, Performance, Presence, Ride, Sail, Stealth and Survival that you can learn.
Granted, you'll likely have 3 of those at 5 already to maximize your points.
Once you can learn charms, I can also see an issue with people doing this:
Learn Craft 5 normally. For covering multiple crafts, there is a charm for that (Supreme Celestial Focus) that should work for Personas as well.
There's nothing prohibiting you from solely spending Persona-XP on Craft-charms. Or buying the basic ones with normal XP and adding to that with Persona-XP.
Similar things can be done with Lore or Occult - or indeed any ability. Have some basic Melee charms, a persona with Melee 5 and learn some additional Melee-charms in that Persona.
I really like the overall concept and with a GM looking for such abuse it's not much of a problem IMO. But yes without that it's problematic.