I noticed some weapons, like the Elven Thinblade, can qualify for Weapon Finesse despite being classified as one-handed weapons. Does this mean I can wield it two-handed for 150% of my Dex bonus? Alternately, how can a Swashbuckler/Rogue multiclass apply Dex to attack rolls?
They are not mutually exclusive
And in fact there's even one weapon that's two-handed but finessable (the Spiked Chain). You can Power Attack with a finesse'd weapon as well, unless it's classified as a Light Weapon. Did the designers intend this? We'll never know, but the RAW is fairly clear on the matter, as there's not text in Weapon Finesse or the section on using a one-handed weapon in two hands that forbids it.