To begin with Indian sub-continent should not be used. I have worn myself out here trying to explain to Paks not to fcukin use terms like India sub-continent, desi, Asian, Muslim and the other synonyms I hear. If we don't own the name 'Pakistan' nobody else will. Specifically with referance to Indian Sub-continent post 1947 that term should have been dropped. These are geographic descriptors and a more neutral term like South Asia should be used. Simply because the term 'India' was branded in 1947 by a nation state called Bharat. Thus to prevent any ambiquity and to be neutral all the states that occupy trhe region South Asia should be used.
With referance to what you say it's not entirely this simple. From tectonic point of view what you said true. But in human geography and even physical geography rivers tend to bind and not separate. Consider Ganga. You can' divide northern plains of Ganga from the souther plains. Similiarly Indus is one physical water system that includes good chunk of Afghanistan. Kabul River is a tributary of Indus as much as Jhelum is.
Where Pakistan should focus on is Indus River simply because one can see Pakistan is underpinned by it. Pakistan occupies most of the Indus Basin. Pakistan can be used as a synomym for Indus Basin. That is why I use that handle. Some of the Indus Basin does extend into Afghanistan, China and India but as the map below shows Pakistan is central to the Indus Basin. In fact in Pakistan you can't really escape from the Indus Basin.
You seem to be a very knowledgeable guy. My regards.