
Fozbek |
1. Does Adamantine armor still grant DR 3/-?
1A. If so, does it stack with the DR/Armor granted by the armor?
Yes and yes. The Armor as DR rules don't say that they supersede the normal rules for adamantine armor, so those rules still apply. Armor DR also explicitly stacks with all other forms of DR.
2. Is there a minimum damage rule somewhere?
If an attack is reduced to 0 or less by DR, the attacked creature takes no damage and does not suffer any effects of the attack that require him to have been damaged. He still is considered to have been hit by the attack, but things like Injury-vector poison won't work. EDIT: This is stated in the definition of Damage Reduction in the Core Rulebook, I believe.
3. Can I also assume that spells that grant AC (such as Mage Armor, etc.) grants DR/Armor when cast by the spellcaster?
Yes. Anything that gives an Armor bonus to AC will instead give an equivalent amount of DR. Anything that gives an enhancement bonus to the Armor bonus to AC (magical armor or the magical vestment spell) will give both a bonus to Defense and DR.

Fozbek |
At what level? 14/- is basically automatic (under this system) for any heavy armor wearer that can afford adamantine full plate. 9 for the armor, 3 for adamantine, 1 for an amulet of natural armor +1 which is chump change by the time you can afford adamantine full plate, and at least 1 for the every-5-levels boost.
Keep in mind that grappled and entangled creatures halve their armor DR (so 14/- would go to 8/-, thanks to the 3/- from adamantine not being halved) and that creatures of sufficient size overcome the DR automatically.