I bought Expedition in 2012 and I am still on a steep learning curve. Practice, the help file, the webinars, Will Oxley's book, and this forum have all contributed significantly to whatever limited proficiency I have acquired. I will try to address some of your questions and I hope that my explanations can be supplemented by others and corrected if my understandings appear flawed.

​Nav Polar: This should be the MH polar when buoy racing as it is used to calculate laylines. If no MH polar is available as the Nav Polar for buoy racing then use the 10M polar as the Nav Polar but scale the polar up to the MH velocities that you will encounter, using the method / formula set forth in the Help file. This should be the 10M polar when routing as you will be using 10M winds from the grib file. If no 10M polar is available as the Nav Polar for routing then use the MH polar as the Nav Polar but scale the grib winds up to MH velocities, again using the method / formula set forth in the Help file.

Perf Polar: This should be the MH polar to be used to monitor boat performance in real time. If no MH polar is available as a Perf Polar then use the 10M polar but scale it up to MH wind velocities that you will encounter, again using the Help file to obtain the scaling method / formula.    

​Note: It follows from the above that it is often necessary to switch scaling percentages in the Nav Polar during a point-to-point event when using the Nav Polar for both routing and predicting time to laylines. This can be obviated by using two PCs with one dedicated to the performance monitoring and a second dedicated to routing and Race Schedules. "Commodity level laptop" is a term I first encountered here on this forum (Thanks JB) and it really is germane to utilizing Expedition in the ocean racing circumstance.

​Heel Polar: Used to generate / display polar / target heel as a value that should not be exceeded in heavier breezes or possibly aspired to in very light stuff. This will not be used in the calculation of leeway; only the real time heel measured by the heel sensor is used as a value in the calculation of leeway.         

​Leeway: You will need to establish a "Leeway Factor" or "Leeway Coefficient" (often referred to as "K")  that will be applied in the calculation of degrees of Leeway made along with BSP and measured Heel. Obtain a factor by massaging the Leeway, BSP and Heel (from the USS VPP data) for your boat in the leeway equation that you can get from the Help file. You will not obtain a uniform value for Leeway Factor throughout the range of upwind sailing but just try to get an average or "most relevant" number; I prefer to emphasize narrower TWAs here. Don't sweat the values with the board up as you will be going too fast by then for leeway to be big enough to significantly influence your calculated TWAs. You will enter the Leeway Factor into the calibration of your instrument system which will use it to calculate degrees of leeway made which will be added to your TWA by either your instrument system or Expedition; depending on what Expedition receives from the processor. Alternatively, you should enter your Leeway Factor into Expedition if your instrument system is not capable of generating TWAs using leeway. 

 I hope this helps and does not tend to confuse.         

To establish a K factor for leeway of my boat I sailed it on a fresh water lake without current. Then I recorded what the instruments gave back for set and drift.  By trial and error I then settled down on a K factor that minimized drift/set (as set/drift is calculated after correcting for leeway).
Did anyone else use this method and how did it work out for you?
 

ZeeZee's method is a great way to go. 
​When I search for the best K value on the Great Lakes I massage K until Course (HDG + Leeway) = COG, after looking over the side at something which can establish that there is no wind-driven set & drift. 

A further question: on the light wind side... 
We just spent 6 hours racing in the "Three Bridge Fiasco" where the winds were max of 10 kts and more commonly under 4kts. 
The standard US SAILING VPP had 6 kts as the minimum TWS.
There's interpolation between TWS values, and 0 kts at 0 TWS is an obvious point.
Is there any value to having more points between 0 & 6  ? 
Specifically, how many more TWS steps make a difference for predicting Target angles/Speeds and routing when the wind is light? 

is "3" kts sufficient or do I need 2 & 4 ?  
I can use tests and take the data to a curve fitting program to extract those values.