I agree with everything you said except the bold. Supporting JI was a bad idea. They delivered in the past during Musharraf's era because the 18th amendment didn't exist and because Musharraf created the local government laws. Now that neither are a reality, supporting JI is as good as voting for PPP.
Sindhis are not the indigenous peoples of Indus River Valley. They have "aryan" DNA admixture and are outsiders just like any other. Baloch too are Iranic groups and don't originally belong to the land they claim.
For Muhajirs, the only solution post 18th amendment and lack of local government is to have the federal government force its mandate through federal projects in Karachi using federal funds. The other way is for the federal government to declare emergency and use Governor rule in Sindh. Whether this means supporting PTI or even PMLN, this is the only help Karachi can get. You can forget trying win elections anywhere in Sindh as PPP will rig them.