Is the Aldori Dueling Sword viable as a Swashbuckler weapon

I would allow it as a GM.

Strict-RAW, the answer would be yes; the weapon is still a piercing weapon even if you are doing things to change the damage you deal with it. Weapon Versatility doesn’t change the weapon, it changes the damage you deal with it, so the weapon is still compatible with the swashbuckler features.

But then, I would also allow a swashbuckler to use Weapon Versatility with a non-piercing weapon to use those class features. Actually, I would probably allow a swashbuckler to use any weapon he or she likes with those class features.

And I would do that because the swashbuckler is an extremely weak class that needs all the help it can get. And ultimately, that is the most important consideration to me: what makes for the best game? The limitation here is not adding anything, and is potentially frustrating an already-weak class. Expanding the capabilities here is thus an improvement to the game.

No, it doesn't work.

But with the Weapon Versatility feat (requires Weapon Focus) from the Undead Slayer's Handbook, you can use the Estoc with Slashing Grace just fine.

Benefit: When wielding a weapon with which you have Weapon Focus, you can shift your grip as a swift action so that your weapon deals bludgeoning, piercing, or slashing damage instead of the damage type normally dealt by that weapon. You may switch back to the weapon’s normal damage type or another damage type as a swift action. If your base attack bonus is +5 or higher, using this feat is a free action instead.

However, there are other options that you can look into:

• Mythic Weapon Finesse - only available if you have mythic. Allows the character to add dex to damage to all finessable weapons.

• 4th level of a swashbuckler archetype called Whirling Dervish. This applies to all weapons that work with Swashbuckler finesse plus scimitars.

At 1st level, a swashbuckler gains the benefits of the Weapon Finesse feat with light or one-handed piercing melee weapons, and she can use her Charisma score in place of Intelligence as a prerequisite for combat feats. This ability counts as having the Weapon Finesse feat for purposes of meeting feat prerequisites.

• 3rd level of Unchained Rogue (and again at 11th and 19th levels) allows one to select a single finessable type of weapon so you can use dex to damage with that type of weapon.

Finally, if the Path of War from Dreamscarred Press (third party publisher) is allowed, you have Deadly Agility, which allows you to use dex to damage on any light or finessable weapon.