what about C6 and/or E6 firing into D4? I assume that LOS is blocked in both cases because there is a bit of wall from the firing building that runs thru B5-C5-D5-E5. Sound correct?
Correct. The LOS is blocked because the LOS touches a depiction of an obstacle (building) in an intervening hex. For example, for the shot from D4 to C6, it is the building on the C5-D5 hexspine. I mention this as WALL is a specific terrain feature in the game, so it is best to think that the LOS touches BUILDING rather than a specific graphic depiction of a building.
2) G3 can fire at I3 because there is no building along the hex line of H3...correct? But C8 cannot fire into E8 because D8 is also building (albeit the same building)...correct?
3) Finally, C5 is an interesting hex (and very confusing to me). It is a building hex for sure but if you are in that hex, you could be in 2 different buildings as there are buildings on both sides of the road in that hex?!?
It doesn't matter (in game terms) which building you are in. You are in a BUILDING hex but the available cover is modified by the presence of the road to 2. Remember that LOS is from the center of a hex and only terrain in intervening hexes (hexes that are neither the firing nor the target hex) impacts the LOS. So am curious what is confusing you.