The chunk probably keeps the pommel on, like a set screw.
I don't know. For all I know, the booster could extend into the pommel but from what I see in the pictures and what was said about the pommel fitting into the booster with flanges, I don't think so. Everything below the booster is a separately machine piece (you even see a separation groove under the booster).
I think the two likeliest theories so far, for me, as to what it does/is, is that it's a plug for a hole and the other (which I hold onto) is that it's a wart. Do we even know that it's definitely a separate piece from the lightsaber? As in it was added to it rather than something that's a part of it?