[Jack of all Trades]

If 75% to 80% of the nucleus is empty, what makes up the mass?

If the comet is a loose ball of powder rather than a hard lump, what holds it together as it arcs around the sun?

Glad to hear some results are finally being published.

[Drammach]

If 75% to 80% of the nucleus is empty, what makes up the mass?

The other 20% to 25%..
It's a small mass, but mass, nonetheless..

If the comet is a loose ball of powder rather than a hard lump, what holds it together as it arcs around the sun?

Just a guess here, but I would say electrostatic charge..
Through ionization, the particles have "clumped" together, and are held together by "static electricity"..
Just like a dust bunny..