I don’t understand your first statement. You apparently believe that the Earth was once smaller, but now you seem to be contradicting yourself. As to your second point, no one is saying that the isotopic ratio is a product of melting or cooling. What they are saying is that when considering isotopic ratios as a method of dating rock, the parent/daughter ratio needed for consideration is from the most recent melting/lithification cycle.
No. Actually I don't believe anything because I don't know. But I do know that anyone who believes that the earth is some accretion of stuff has to believe in a 7000 mile diameter earth. It probably has a bit to do with my belief in the correctness of the math I learned and something called the Intermediate Value Theorem. Of course if the earth came about more like something we read in Genesis then I supposed the Intermediate Value Theorem wouldn't apply.
And as for isotope ratios, I suppose I am saying that I think state changes of a substance have noting to do with the isotope ratios exhibited by that substance and so deserve no consideration. I cannot tell whether you are being purposely dense or I have lost the ability to express myself clearly. Perhaps others will weigh in.
ML/NJ