I also think the developers should publish more details about their priorities.

However, there's already some information about the link between Mijin and NEM, see the Catapult Whitepaper:

https://nem.io/catapultwhitepaper.pdf

Basically with Mijin you can use private blockchains while NEM offers a public blockchain.

With the catapult update, private blockchains can be anchored in the public blockchain (see the following link, paragraph 2):

https://blog.nem.io/nem-mijin-blockchain-technology-briefing-january-2017/