Prefabricated wood burning fireplaces often come with air intake vents of the kind you are considering installing. I would check the specs on 2 or 3 of those and see if the location and size of their air intakes give you a clue for your installation. You might also pose your question in the forum in hearth.com. That one is staffed by guys who live and breath fireplaces.

inside air, outside air, its your choice and with your situation either would probably be just fine, you could take it from the garage, but if it were me, i would not, and not because of any really good reason other than if i were going to bother running pipe, then i would run it all the way.

Without discussing the benefits of venting for different systems, or whether it is needed, http://presbyeco.com/technical-information/the-importance-of-venting/ Check to see if something like "by-pass" venting will work. The septic tank is generally required to be at least 10' from the house. Also, do not forget that a house has operable windows, so any minimum distances that may not have been followed would be more likely the fault of the septic system installer, and not the fault of the person that installed the fresh air intake.

I think the short answer is "it depends". Our fresh air on the existing house was specifically put somewhere where it was NOT near any kind of electromechanical requirement, AND not near car exhaust AND not near lawn to try to find the best place that would have the least amount of chemicals and potential pollutants being brought back into the house ostensibly to provide "fresh" air. Having this done in a thoughtful manner is more and more important when the rest of the house is tighter and less leaky. From Building Science: Ventilation System Intake A supply-only or balanced ventilation system offers control over the source of dilution ventilation air. This is a considerable advantage if the outdoor intakes are located where they will not likely entrain contaminants, dust, debris, odors or other substances that would degrade indoor air quality. The following rules are offered to govern location of outdoor air intakes. Intake must draw through an intake grille or register located on an outside wall or soffit and not the roof. Wall intakes must be located at least 10 feet from any appliance vent or any vent opening from a plumbing drainage system. Wall intakes must also be 10 feet from any exhaust fan discharge outlet unless that outlet is 3 feet or more above the intake location. (IRC 2006, Section M1602.2). Intakes should not be located directly above any wall exhaust or vent. Intakes must be protected against rain and pest entry with durable rust- and rodent-proof materials. Intakes must not be located within 10 feet above any paved surface or roof. Intakes must not be located in proximity to an area where vehicles or motorized equipment is likely to idle for any length of time. Intakes must not be located where they might be obstructed by drifting snow (consult local experience as to the height of snow drifts against buildings). Intakes must not be located within a zone of significant ground-level dust and debris generation (e.g. mowing, leaf blower use). At a minimum, intakes should be 2 feet above grade. Intakes must not be allowed to be obscured or hidden by vegetation. Vegetation must not be allowed to grow within 3 feet of a ventilation system intake.