While you are correct in saying that the WMDR is intended for unlimited travel across a certain area, I have found that (from my own experience) when travelling on a Wigan-Birmingham Off-Peak Return costing £23.70, it is cheaper to instead buy a Wigan-Crewe Anytime Day Return for £8.40 and use a WMDR ticket from there costing £11.10. Buying Advance singles for Wigan-Birmingham come to around £18-19, but you lose flexibility, so for the same price you are better off using the Wigan-Crewe/WMDR combo if you are returning the same day. And now that it can be used on Virgin it eliminates the need to change onto LM services at Crewe, thus making it more sense to save the £4.20 by going down this route.
All prices above are with 16-25 Railcard discounts. It is only an example and I'm not sure what the differences are without the railcard discounts, but it shows that if you do your research some modest savings can be sought if you are travelling long distances to/from the West Midlands for the day.
With respect, I think you were missing the point I was making.
For your journey, you are using a WMDR, quite legitimately, to travel between Birmingham and Crewe, and are then buying an onward ticket to Wigan your ultimate destination. You are not attempting to use the WMDR outside the area it covers. That you are using it because it makes up part of the cheapest ticket combination isn't an issue.
The previous posts were querying if you could use a WMDR to travel from Northampton (which is the southern boundary for a WMDR) to Stoke (also within the WMDR area) travelling via Milton Keynes (which is NOT in the WMDR area).
Easements to allow "turning back" surely cannot apply since, as I previously stated, the WMDR (or any other Rover or Ranger for that matter) allows for travel within an area, it is not specifically defined between 2 points. So you can travel anywhere within the prescribed area, you cannot travel outside of it.
However, if you've bought tickets for a specific journey e.g. Northampton to Stoke, then the easements may apply - in that instance depending on the operator and tickets you've bought.