The answer to your question requires some deductive logic. Let's start with (1) all bodies are extended, which is Descartes' premise. It follows logically that if something is not extended, then it is not a body. Thus (3) follows logically from (1). You are worried because you think that (3) assumes that something is not extended. But it does not. It only claims that IF something is non extended then it is also not a body. So physicalists can agree with the claim.
Here is an argument that determinism doesn’t undermine, butenhances, free will.
(1) Our actions are caused by our propositional attitudes,such as desire, hope, acceptance and belief.
(2) The more deterministic the relationship between out attitudesand our actions, the more freedom of will we possess.
(4) The more control we have over our own attitudes the morefreedom of will we possess.
(5) Our control overown attitudes consists in the influence of some of our attitudes over others.E.g. We want to smoke. We also want not to smoke (These are called first-orderdesires) And we want not to want tosmoke and we do not want to want to smoke. (These are called second-orderdesires) We have freedom of the will toextent that our desire not to want to smoke wins out. (From ‘Freedom of theWill and the Concept of a Person’, Harry Frankfurt, The Journal of Philosophy,1971).
(6) The more deterministic the relationship between oursecond-order desires and our first-order desires, the more freedom of will wepossess.
(7) Determinism is irrelevant to freedom of the will in allother respects. It doesn’t matter how our attitudes got there – whether byrandom processes or deterministic ones, they are as they are. And we want themto be in control of our minds and our bodies – for self-management and managementof the external world, as far as possible.
Good self-management – looking after your own desires,emotions and reactions to things is a healthy Stoic philosophy. If you feel yourselfgetting angry and resentful ask yourself why you feel this way – for example: isyour pride affected, or your self-esteem or do you feel threatened in someother way? Ask yourself whether you might have done something to bring aboutthe situation that angers you. Ask whether realistically there is somethingconstructive that can be done to rectify matters. If vengeful thoughts arise,recognize them and banish them. No good can be achieved by vengeance. Harm toyourself would result fro, any attempts at revenge. If there is somethingconstructive to be done, decide whether to do it. If you decide to do it, doit. If you decide not to, let the matter pass and move on. Consider that youyourself would prefer peace of mind than the disturbance of the anger. In thisway you can exert some control over your own mind and attain a more sereneexistence.
Also I have heard tell that quantum laws fix the probabilityof any event’s occurring. No self or soul or will can affect theseprobabilities without violating physical laws.
It may help to say that we only need "infinite divisibility"--we don't need the division to have actually taken place, only to be possible in principle.
Also, what is your model of "dividing" here? Do you imagine scissors and paper and a lot of cutting? Perhaps there are other ways of conceptualizing infinite division that don't require time.