Before my conversion to Catholicism, I read that God said, "If you love me, keep my precepts." Do you know where in the Bible I can find this? Is "commandments" another word for "precepts"?
You are referring to Jn 14:15, "If you love me, you will keep my commandments". Now, "commandments" and "precepts" are two possible translations for the Gk (ἐντολή, entole), but in Jn 14:15, it's almost always "commandments" or "commands". In fact, there isn't a single English translation I'm aware of that uses "precepts" in this verse.
When did folding or joining your hands together become the norm during prayer?
Pope-Emeritus Benedict XVI gives us this explanation in his book Spirit of the Liturgy:
A later development was the gesture of praying with hands joined. This comes from the world of feudalism. The recipient of a feudal estate, on taking tenure, placed his joined hands in those of his lord -- a wonderful symbolic act. I lay my hands in yours, allow yours to enclose mine. This is an expression of trust as well as fidelity.
[. . .]
What within feudalism may be questionable -- for all human lordship is questionable and can only be justified if it represents and is faithful to the real Lord -- finds its true meaning in the relationship of the believer to Christ the Lord. This, then, is what is meant when we join our hands to pray: we are placing our hands in his, and with our hands we place in his hands our personal destiny. Trusting in his fidelity, we pledge our fidelity to him. (pg. 204-205)
Theoretically could the church open the canon of the bible and add books if it wanted since she determined the canon in the first place?
No. The canon of the bible constitutes public revelation, which is closed. From the Catechism we read:
66 "The Christian economy, therefore, since it is the new and definitive Covenant, will never pass away; and no new public revelation is to be expected before the glorious manifestation of our Lord Jesus Christ." Yet even if Revelation is already complete, it has not been made completely explicit; it remains for Christian faith gradually to grasp its full significance over the course of the centuries.
Did the devil and Michael really fight for Moses's body or was it a metaphor for something else?
The dispute between Michael and the devil over the body of Moses (cf. Jude 1:9) derives from a Jewish apocryphal work called The Assumption of Moses. Since it is difficult to determine what from an apocryphal work is truth and what is "pious legend", it's difficult to say if this really happened or not.
The historicity of the event is really beside the point. Jude referred to a work that his audience was familiar with in order to prick their conscience. The archangel's prudence in not even cursing the devil serves to highlight the arrogance of the people, who cursed the angels.