My suggestion is to have the gallant swashbucklers add their CHA mod to their AC as long as they are not using a shield or offhand weapon.
1. to incentivize no shield swashbucklers
2. to match their description of " excels both in combat situations and social interactions, making her a versatile character indeed. "
Currently the split between STR/DEX/INT they require makes CHA a tough choice to take without some merit though I do think it would fit the class fantasy.
If it could also be used to prevent shield use which in my opinion does not fit the class fantasy that would be cool.
I think both un-buffed and buffed mod might work but i'll leave that to the balance pro's.
Unbuffed charisma to Shield AC as long as they have an empty off-hand seems like a good choice.
That said, buffed AC is not likely unreasonable either. It would bring Swashbucklers close to Monks in terms of buffed AC.
I'm assuming the likely scenario is +3 AC from a shield vs buffed CHA of +4 to +5 at most.
More likely, a Swash with 12 charisma is going to have at most+3 AC buffed without a shield - unless they get Aura of Glory reliably.
Making it scale off CHA directly seems like a bad idea. Perhaps instead something like the following, since level 8 is a bit less of a jump than it once was as well:
QuoteGrace is now disabled if you are holding a shield.
Grace now gives +1 reflex and +1 shield AC per base CHA modifier. These bonuses cannot exceed your Grace level.
Grace +2 at Level 8 Swashbuckler (was 11)
Grace +3 at Level 11 Swashbuckler