Mijin, I am told, does not actually use xem itself, however if the private chain of Mijin had need to communicate/transact outside of the private chain, it would use the public chain Nem.
This is just one thing I have heard yet I would not put it down as a carved in stone objective.
I look at Mijin as a seperate entity to Nem chain.
However if the same devs work on both and Mijin
is being tested, personally just at a guess I would say this makes for a pretty good foundation in the future, wether or not Mijins use will actually end up stimulating the Nem network and/or adding value to the actual xem coin, seems like a grey area at present.
At a guess I would say Mijins aim is a bit broader than adding price value to the Nem network and xem.
In general many possibilities could arise in time.
As for github I do not think much of Catapault is open source …yet.
Not even sure if Mijin ever will be, yet Catapault is meant to be open source soon, so I guess Mijin is part of that…
What I posted is pretty much old info, the newest being around 3 months old.