Easily sorted out, order 172s but fitted out for outer suburban/regional duties with tables, first class etc... then diagram them on the Hereford/Shrewsbury workings.
To differentiate between the 172s, just have 172/0s on the Chase Line workings and have the 172/1s working the Hereford and Shrewsbury workings.
That could be done, but let me just correct you on your ideas:
172/0 - 2 car units; operate on London Overground services; no gangways; surburban layout.
172/1 - 2 car units; operate on Chiltern Railways services; no gangways.
172/2 - 2 car units; operate on London Midland services; gangways; suburban layout.
172/3 - 3 car units; operate on London Midland services; gangways; suburban layout.
I would first order the add-on order of 172s for LM, to increase capacity on the Snow Hill Lines. Then I would order a few more 172s (like existing LM 172s) to displace the 170s on the Chase Line.
Then we have the Birmingham-Hereford/Shrewsbury services: I would order a new batch of 172s which would have the same interior layout as a 170 and to allow for future increase in passenger numbers and alleviate overcrowding, they would be 3 or 4 cars long (and possibly implementing a small 1st class section by one driving cab). They could be designated class 172/4.
Or, we could go a bit further and have a separate class (class 173?) that would be the same as mentioned before but the layout would have end door layouts in stead (à la class 158) due to the longer distance nature of these routes. They would still be able to work in multiple with 172s and 170s, having end gangways. This new class could also potentially be ordered for FGW, to allow them to cascade/rearrange their 158s.
This would allow LM to be rid of all their 170s which would then be cascaded to other TOCs.