Farhan Bhai,
As per reply given by aesiege in post number 214, 217. It is evident that we can reach frm Kala pani to Rattu and then astore. It mean yes it is doable.
As per aesiege
"There are 3 contributory valleys that make up together Gagai Nallah.
All three lead to 3 different treks from Kashmir to Baltistan around Minimarg and Kala pani region:
1- Taobat - Kamri pass - Minimarg
2- Taobat - Kala pani pass - Kala pani
3- Taobat - unnamed pass - Finishes at villages between Kala pani and Chechri Kadal
The route coming from Astore divides into 4 valleys:
1- Chillam valley
2- Rattu and Kala pani
3- Rattu and Shounter side
4- Trashing and Rupal side "