Now we can understand the Ran who explained the drasha of "מקצת זרעו ולא כל זרעו". He said that the Torah only forbade giving some of your children to Molech but not because that was the way they used to serve the Molech ["דרך עבודתו"]. They would only give some of the children but not all. [עי' גם בחינוך מצוה ר"ח ובמהר"ל בבאר הגולה באר השני].
This apparently makes no sense! If this is the case then why do we need to derive this from a pasuk? The Gemara says that if Molech is not classical Avoda Zara then one is only חייב if he served Molech in the accepted way and no other and so paskened the Rambam [Avoda Zara 6-3]. So if it not the accepted way he will automatically be פטור. So why do we need a pasuk to tell us that he is פטור if he gave all of his children to Molech?
We MUST say that the Ran holds like the Kesef Mishna and not like Tosfos. So without the pasuk we would know that he is פטור when he give his only son or gave all of his children at the same time because that is not the normal way to serve the Molech. The pasuk comes to teach that if he gave his children to Molech one by one and eventually gave all his children he will be פטור. Even though when he gave the first ones it was דרך עבודתו and he should be חייב, nevertheless he is פטור because the according to the Torah there is a צירוף between the different children because they constitute in toto the continuation of the generations and when passing them all through the fire he doesn't deserve a כפרה [as we explained previously] and is thus פטור.
