Yes, dopamine receptors (D1-D5) are G protein coupled receptors, which act primarily on adenylatecyclase (changing cAMP concentration, which activate proteinkinase A, which phosphorylates other proteins in turn such as CREB... you can find the pathway in the following article). D1 receptors activate adenylatecyclase, D2 inhibit its activity.
I wouldn't say it directly alters the Ca2+ channel concentration per se, the article says however it can modify their state.
According to this article, G proteins of these receptors could also have different effects (alternative effects through G-proteins such as activating phospholipase C, which results in IP3 formation which results in Ca2+ influx to cytoplasm from ER which acts on proteinkinase C) or new research suggest that it could also act through G protein-independent mechanisms via interactions with ion channels.
specificity of the location (of the brain) that dopamine acts on controlled?
The idea is that cells with certain neurotransmitter may induce growth of cells with same transmitter, thus forming specific circuits. I don't know however how specifically this happens and how the cells communicate to form specific connections (like dopaminergic connection from substantia nigra to basal ganglia) during development. I'd bet on some morphogenes. Someone else might know better :) .
Yes, dopamine is associated with addiction, as it's part of the reward system in brain. Many drugs inhibit its transport from synaptic cleft into the cells, which means prolonged activity, brain senses positive effects you can see when you've done something good (reward).