To access an equivalent amount of mass from the moon you would need to strip mine it to a depth of about 23.5km. On earth it would be about 1km. Your point about losing a lot of it as reaction mass still stands, but 4% of the moon is actually quite a lot.
Yes, and it is spread out over a huge amount of space. The asteroid belt occupies 1AU of space that starts 2.2 AU out and and ends 3.2 AU out.
Sure, roughly half of the belt's mass is contained in just four asteroids - Ceres, Vesta, Pallas, and Hygiea.
Ceres alone is the 33rd largest known object in our solar system and simply mining it might be realistic but its orbit is 2.5-3 AU out but from what we've observed it is estimated to be 20% carbon by mass in its near surface, that is a LOT of material that you have to remove just to get at other stuff. With a (believed) rocky core, you have 70-190km of ice that likely varies considerably in composition and concentration.
Even if you can get out there, even if you can mine it at -36F in direct sunlight, you're going to have a world of trouble doing it and may not even find a worthwhile concentration of materials you actually want, you may be processing ice and dust attempting to skim the rare earths and precious metals out of the dust.
