Not only that, but excluding the four giant planets isn't exactly helping your cause. Over 99% of the planetary mass in the Solar System is tied up in Jupiter, Saturn, Uranus, and Neptune, it looks an awful lot like you're cherry picking data here. If your theory is true, it should hold for every body in the Solar System, regardless of size.

The six minute discrepency between the two sets of measurements of Saturn's rotation isn't anything to get too worked up about. It's only a discrepency of 1% and well within acceptible errors, particularly when you consider that the instruments on Cassini are some 20+ years more advanced than the Voyagers'.

Molecular bonding has little to do with how a planet holds together.

Neptune doesn't look anything like the inner planets. It's a honkin' big ball of hydrogen and ammonia. The inner planets are, essentially, rocks and iron. Its density is much less than the Earth because it's a ball of gas instead of a ball of rock with a creamy iron center.

Neptune's satellites also look nothing like the inner planets. Amongst other things, you don't find volcanoes that produce liquid nitrogen in this part of the Solar System. Their density is much less than the Earth's simply because they have different compositions - most notably, they don't have a large iron core like the Earth does.

You're right that volcanic activity is taking place in the outer Solar System, or at very least has taken place in the recent past, but I don't see what this has to do with your theory. Tidal interactions between moon and planet provides a perfectly good explanation as to how this occurs.

Edited to add a missing word.