1. One standard cargo hold will hold one mine, one automine, or or mass driver. You can estimate how many installations one trip can transport and use repeating orders to order a desired number of mines be moved. Or you can use civilian shipping assets once they start building cargo ships.
2. Free enterprise enters into the equation when you start getting civilian mining complexes (CMC):
http://aurora2.pentarch.org/index.php?topic=1716.msg16335#msg163353. Per annum means per year. But it actually works as per year per mineral type. So an asteroid with Duranium, Sorium, and Gallice will produce 14 tons of each, multiplied by the local accessibility multiplier. Personally, I deploy 50- or 100-module engineless mining rigs via tug. The real value in asteroid mining operations, if you ask me, is in cranking out a constant stream of mining assets without occupying my construction factories.
4. Uses for a planet without minerals include:
-populating the body to generate wealth, improve population growth, and distributing the manufacturing sector across multiple planets
-building a rest stop for passing ships to refuel, resupply, load ordnance, conduct shore leave, and/or undergo maintenance overhauls
-constructing armed PDCs to defend the area, if desired
-exploiting research anomalies, if present