Fullness of the Gospel
 

1. What does it mean?
2. Is it contained in the Book of Mormon?
3. Is it contained elsewhere?

Conferte.

The "gospel" is one thing.
3Ne 27:13,14

13 Behold I have given unto you my gospel, and this is the gospel which I have given unto you—that I came into the world to do the will of my Father, because my Father sent me.

14 And my Father sent me that I might be lifted up upon the cross; and after that I had been lifted up upon the cross, that I might draw all men unto me, that as I have been lifted up by men even so should men be lifted up by the Father, to stand before me, to be judged of their works, whether they be good or whether they be evil—

Everything points us to this.
To think that the things that point us to "this" is the gospel is tantamount to looking beyond the mark.

2. Yes.
3. Yes.

Both the Bible and the BOM contain the Fullness of the Gospel.

So then what does fullness mean? Does it mean the complete gospel? Because neither of them contain the substance of the temple rituals which are necessary to return to the presence of the father.

What exactly do we mean when we say this? If we mean "complete" I don't know if I can agree that anyone book contains that.

I think you partially answered your own question. "Fulness" cannot possibly mean that each book contains everything about every ordinance, etc...at least not in their respective present incarnations--because neither book contains all of it.

But what is the purpose of ordinances?
DC 84:20:
20 Therefore, in the ordinances thereof, the power of godliness is manifest.

God keeps giving us ordinances because we keep missing the point. Like the Israelites of old. Nothing ever changes.

The power of god is his ability to change a fallen person. atonement. The purpose of ordinances is just to try and help us think of that.