Would Earth's diameter be less if the current liquid mantle turned solid all the way to the core?

i think you answered the question yourself! the solid would be more dense and take up less space, so the diameter would probably decrease.

and yes, as distance decreases the force increases, so we would weigh more, but our mass would be the same.

Actually, I believe solid matter occupies more space than liquid matter. But if the earth's core were to solidify somehow, resisting the crushing force of gravity trying in vain to start a fusion reaction...I suppose the Earth's diameter would actually increase, and gravity would be less, due to the increase in radius without an increase in mass.

But that'll never happen because the melting point of the minerals at earth's core is far too low to resist the heat produced by the gravitational forces within it.

That is a good question, given that the same mass will occupy a smaller space, i would think that the gravity would be stronger, however, we wouldn't be feeling any gravity because we would all be dead, due to no magnetic field because the core and mantel are solid.

We may not be heavier since the total mass of the earth will remain about the same.

The diameter is most likely to decrease.

hmmm, if it were to solidify maybe diameter would increase coz the centre of the earth would be less dense,

pity einstein aint still around, this is a question for him

Really, you may want to reconsider.... Because water as Ice takes up more space than it does as liquid water, anyone with any basic knowledge of physics knows that..

no liquid does not take more space than solid, just think about your ice trays. you don't lose space when it freezes unless it evaporates. So no to your question

it would cause the crust we float on to get sucked in and the gravity and spin would change and life as we know it would be gone.

the earth's diameter would decrease but we might not notice it

since the earth is pretty rigid i doubt that diameter would change substantially enough for any change ...