Terra's Big Neighbor

[radtech497]

Also, would a somewhat larger planet in Venus's orbit (say, 2x Earth mass) be a problem for Earth's orbital stability?

No, a planet that size in Venus' orbit (assuming eccentricity and semi-major axis remain the same) would only extend the planet's gravitational reach Solward by slightly more than 1 percent (1.013%, or 0.691 AU) and Earthward by slightly less than 1% (0.945%, or 0.755 AU). The planet would, all else but mass remaining the same, probably be a bit brighter in Earth's sky (its radius would be 34.891% larger) than Venus is now, though the cloud cover might be less.

Radtech497