I'm not sure where to start.
I believe you are over-thinking it. A lot.
We do not know how far the wire runs.
A light bulb won't tell anything. They can do funny things used like that.
AWG #15 is listed as 3.2 ohms per 1000'. Don't forget wire goes out and back.
So figure how many feet of wire from the PVs to the battery, double it, calculate the resistance. Most connections have resistance too.
Quick thinking, I'd say you dont want to go more than about 25 feet from the PVs to the battery, if both PVs are going through the same wire you said.
The math is not as straight as it seems at first. It's not linear. I'd say forget about it, read and check the chart in this link.
http://www.windsun.com/Hardware/Wire_Table.htm
Then...
run down to the hardware store, get some sunlight resistant 12-2 w/g UF-B, and 3 water tight strain relief fittings for 12-2. That wire is a bugger to work with, but worth the effort. Run out of one PV, into the next, then out and down to the battery... so 3 fittings.
My self, I would move the diode to the battery. A big diode, like use 2 terminals of a 25 or 35 amp bridge rectifier. Or 3 terminals if you can figure it out so it uses 2 diodes in parallel. It will be easier when you decide to change over to a charge controller like an SG-4 (regular readers knew that was coming).
A lot of this is a gut feeling and experience.
I will post a sketch of the bridge using 2 diodes if you want it.
G-