Pre-Modern Astronomy vs Habitable Venus

[Cray]

In another solar system, if two Jovian planets are within a few AU's, or a hot Jupiter is close to its sun, would the gravitational forces be enough to make the rotation and tilt vary widely?

I think so.

If a planet has an entirely liquid surface, maybe oceans many kilometers deep, would that make any difference?

Yes. Part of the cause of instability in a planet is uneven mass distribution: uneven continents, equatorial bulges that other planets/moons can get a handle on, etc. An ocean planet would be relatively even and less susceptible.

I think.