Will it blend? Does the following combo work? Please explain why it does or does not; I want to make sure I get it right before incorporating it into my build.
Take the feat Slashing Grace (longsword) with a daring champion cavalier. Be sure your longsword has an effortless lace attached to it.
Add your Dexterity modifier plus twice your cavalier level (from Challenge and Precise Strike) to your weapon damage rolls. Also benefit from Weapon Finesse and all your class abilities, even though it is a longsword, and dual-wield said longsword with a spiked heavy shield while taking minimal TWF penalties.
Yes, sort of. The core trick (Dex to hit and damage with a longsword) will work because the Effortless Lace only makes the longsword light for the purpose of abilities that can be used with light weapons. Since Slashing Grace cannot be used with light weapons, the longsword will still count as a one-handed weapon for it.
However, you can't use Precise Strike, because:
"To use this deed, a swashbuckler cannot attack with a weapon in her other hand or use a shield other than a buckler."
The ability to get dex to damage and treat the off-hand longsword as a light weapon will work. It is not really any different than the idea of using a sawtooth sabre that is popular here.
As for the double damage, it works for now, but due to the editing errors in the book I don't know if it will stay working that way.
Short version: I don't see any holes in your idea.
Alright, so it does a little bit less damage when the shield is used (no Precise Strike), but it all works fine otherwise?
Alright, so it does a little bit less damage when the shield is used (no Precise Strike), but it all works fine otherwise?
Yeah it works.