OK, I think I have this figured out and am posting for future generations.
It's a case of something a lot of people knew, but no one has posted a *complete* answer.
To set the front of the lot on a MAC (which may be different from Windows and part of my problem) you have to turn on "testingcheatsenabled true", go into build mode (either through town edit or during gameplay by clicking the ground and selecting "build on this lot"), hold down Shift *and* Control keys while clicking the ground (not the building or a door) on the side of the lot you want to be the front, and select the "set new front of lot" option.
Besides the control key detail, here is the missing bit - Not only are you setting the lot front, the blue arrow is the exact spot where the Sims enter, so click as near as you can to the door you prefer if you have several and make sure the bouncing arrow shows up there. I still need to gameplay a bit to make sure I'm 100% right, but so far it's worked like a charm - the lots I was having trouble with had the correct lot side, but the arrow was way back in the middle of the lot, not at the corner I was trying to herd the Sims toward.
Set Front of Lots - Click to see larger image
Thanks everyone, for your input!