A really good set of questions.

I'm sure the delta-V required is lower for reaching an Apollo or Aten asteroids than the Moon or Mars. But exploring and exploiting one is a long duration "mission" ( a year or two) and people have shot themselves in the foot by calling Mars (a long duration mission) "a bridge to far" thus inaccessible for years. We need to learn how to be away from Earth longer than 6 months they say...

How many years in orbit is ISS celebrating this week...five?

The Apollo asteroid Ed Grondine was on about back a couple weeks (1998 KY26) has a very low relative motion when near Earth, but seems to be mostly rock and water. Still an Asteroid mission would be a good idea. I think the wealth of materials on one should be explored and exploited if possible.

I haven't read my copy of John Lewis' "Mining the Sky" (packed up?) But I imagine it has the details you are interested in.