Swashbucklers and dueling swords.

1) With the swashbuckler's "Swashbuckler Finesse" trait, would I only have to get weapon focus with the dueling sword for slashing grase?

2) Is it worthwhile?

1) Yes that would work since Swashbuckler's Grace gives you Weapon Finesse for the purposes of meeting feat requirements. I'm also fairly certain Slashing Grace was designed for this very purpose.

2) Sure why not. Grab up the Swordlord Prestige class for capitalizing on that dueling sword. And Sword Scion Trait from Kingmaker for additional giggles.

1) Yes that would work since Swashbuckler's Grace gives you Weapon Finesse for the purposes of meeting feat requirements. I'm also fairly certain Slashing Grace was designed for this very purpose.

2) Sure why not. Grab up the Swordlord Prestige class for capitalizing on that dueling sword. And Sword Scion Trait from Kingmaker for additional giggles.

how well would that class work with the Swashbuckler? I'm moderately unfamiliar with both of them. Also: does deft strike stack with slashing grase?

Unfortunately those two abilities won't stack. No adding ability scores more than once. But Swordlord does have a couple other nifty abilities and at the very least might make a decent 1 level dip just to get the free Dueling Mastery feat.

Higher base damage than p.e. a scimitar for 1 feat, but lower crit=lower panache regain.

If you are going to get an exotic weapon, I would look at katana or falcata myself.
Katana=more crits higher damage than scimitar
Falcata= more crit damage, less panache

Keep in mind, that a lot of your damage is precision and not multiplied.

As a swashbuckler, the benefit is that you don't actually need a finessable weapon. I'll leave dueling sword for non swashbucklers who want to use slashing grace