Well, of all the points I think L3 would be stable the longest since the attraction between planets is minimized. Over time the extra force might cause them to spiral inwards, but the effects of other planets would destabilize things more quickly.

No, the planets would probably end up orbiting each other or colliding. Lagrange points are stable for things like satellites because their masses are insignificant compared to the Sun and Earth. If an Earth-sized planet was at one of the Lagrange points(mostly L1 and L2, I’m not sure about L3-5), the planets would be attracted to each other much more than to the sun.