Presence

Yesh miYesh vs. Yesh m'ayin was undecided at the time of the Rambam. It was, to the best of my knowledge a machlokes rishonim. (IIRC the Ralbag held yesh mi'yesh It's also worth noting that the Raavad chides the Rambam on the issue of the incorporeality of God. If I remember correctly the Raavad wrote (the the effect of) that better men that he (the Rambam) believed that God was corporeal. The Raavad wrote (IIRC) that they were wrong for believing that but that it wasn't heresy.

Yes. The Rambam considered the 13 items to be absolutely indispensible tenets. Other beliefs might be important, but were not indispensible in his view.

On the issue of corporeality, the Raavad was disputing not the contention that God has no body, but the Rambam's assertion that it is a "deal-breaker", as it were.

Greg
Rambam does list creation ex nihilo in the fourth of the 13 principles -- not when he first wrote them, but when he revised the Commentary on the Mishnah towards the end of his life. For details you can see M. Kellner's book on Dogma in Medieval Jewish Philosophy. For the revised text see R. Kafih (Heb.: Kapah's) edition of the Perush on the Mishnah. The revision is in Rambam's handwriting in the margin.
What happened? Well, according to my theory, the Rambam did not believe in the importance of creation ex nihilo before he wrote the Moreh. He was perhaps inclined to explain creation according to the philosophical view, the world's metaphysical dependency upon God. After he completed Mishneh Torah he came to believe otherwise. In fact, he stressed creation ex nihilo's importance in four other writings, all dated after the Moreh.
Maimonides changed his position on miracles; before the Guide he interpreted Hazal (e.g. in Avot) as saying that they were programmed into the natures of things, a position he abandons in the Guide and subsequent writings (although he commends Hazal for trying to naturalize them).
All in all, I am arguing in a series of forthcoming papers that Maimonides became somewhat disenchanted with philosophical naturalism before he wrote the Guide, the view that everything that happens happens according to laws of nature. He came to believe that there is a deep division on this point between the philosophers and the Jews. No such division is apparent in the pre-Guide writings.
I don't know if I can count as a Rambam scholar, but my book On Maimonides is coming out at the end of the month, Gd willing. It is published by Wadsworth Publishers.

Dr. Manekin,

Wow, thanks for commenting!

I'll have to go back and look at the sources you mention. My statements about the fourth principle were based on the translation in Dr. Twersky's book; I'll have to check out the Kapah edition. I wish I had Dr. Twersky's translation here; I think it is worded carefully as to allow for Plato's opinion to fit as well.

As for creation ex nihilo in the Guide, it seems that even then Rambam leaves open the possibility of believing in Plato's opinion (I believe this is in Book 2, Chap. 25). He stresses its importance, especially in relation to miracles and their possibility, but doesn't go so far as to require one to believe in it in order to get into Olam HaBah.

I guess where I was going with this was the idea that, perhaps, Rambam considered creation ex nihilo as one of the axioms of Judaism. Unlike the 13 principles, an axiom is merely an assertion, rather than a logically-derived truth. Rambam seems to feel that creation ex nihilo, while perhaps unproveable, is a key requirement for many of the other fundamentals (resurrection of the dead in particular; I believe he mentions this in the Guide, also Book 2, Chap. 25). I thought this might be connected to the different usages of followers of "Moses our Master, Abraham our Father" and a "member of Israel" as stated in Perek Chelek. The former referring to one who believes in the Jewish world view, which includes creation ex nihilo, and the latter referring to those that have sufficient actualized intellects to enter the World to Come.

This is all just speculation, I'm certainly not qualified to make definitive statements on these topics.

Thanks again for commenting (and I would consider you a Rambam scholar). I'll make sure to pick up a copy of your book!