3 phase AC: Can I terminate at main service junction to mitigate line loss? Harmonics?
You are describing a ring circuit where each load on the circuit is fed from both sides. Your analysis is correct.


  
  Would the load equalize between the uneven conductors?
  


The currents will vary to balance the voltage drops along the line, as you suspect.


  
  What kind of effects would this have on the line loss?
  


You will improve line loss at every load point.


  
  Would this introduce any harmonics into the system?
  


No. Harmonics are caused by non-linearities in current draw by non-linear components such as rectifiers or saturated (overloaded) transformers, for example.


  
  What other factors should i could consider.
  


Safety. What happens if the loop breaks and you don't detect it. There are plenty of precedents for this, however, such as doubling up cables for a single load.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Equivalent circuit.

You should be able to simplify the analysis to the circuit of Figure 1 for each phase. If neutral current is high further analysis of that will be required.

There's a sample calculation at The IET forum which may be of help. The Wikipedia article Ring circuit may also be of interest.