I was able to do exactly what you are wanting to do from Fascinate to Fascinate, but yeah we don't have that WiMax.img file. If that's all that needs to be changed, then everything else should work fine.
I was able to do this without a problem on two EVOs with the same hboot and I didn't restore WiMAX.
The MD5 is just generated from the nandroid itself and not the phone so you don't have to change anything in there.
I would just make sure both phones have the same hboot and go for it.
But you also have to consider how much you use your phone versus how much she uses hers. My g/f gets great battery life and just uses her Evo as a glorified feature phone