Lots of questions by an architect for answers contained in the IRC. However,
Providing that the design is within the scope of the IRC...R101.2, I will offer the following answers based upon my opinion....others may vary and ultimately it will the AHJ's call.
Condition # 1.
1. 10 feet from each other... R302.1
2. See Table 302.1
3.Could be, the garage has an allowance for a reduce property setback and allowable projections based upon the exception # 2 of IRC section 302.1.
1. IS this a single familiy dwelling, if so, the IRC does not limit the location or size of your private garage.
2. No overangs across the property line permitted.... see limitations on projections in IRC R 302.1 (code and/or Table)
3. No - R302.1 will tell you the limitations for walls, projections, openings ,etc.
4. See response #3 for condition # 1 above.
BTW, I hope you have a code book.