First of all the Kathak dance system isnt performed in Punjab and hasnt been part of the Indus culture. It mostly received patronage from the Mughals and was based in Uttar Pradesh or Rajasthan. Why dont you enlighten us as to what the 'Indus' dance form is and exactly how it is related to those of the Ganges and Dravidia (South of the Vindhyas) regions?
The main sites of the Indus Valley Civilization, including Mehrgarh (Balochistan) are located in the Indus Valley (hence its name) at Mohenjodaro, Sindh and Harappa, Punjab. Having the North-Westernmost parts of India in it doesnt really change anything. More than 80% of it was based in the Indus Region (Pakistan). Anyway that also excludes the entirety of North, Central, South and North-Eastern India.
The Rig Vedic Indo-Aryans are indeed theorised to have come from Bactria however in the time when the actual Rig Veda was formed (around 1500 BC) it was entirely formed in the Indus (Punjab). Also, it is devoid of much caste references and takes a monotheistic side. Later the center of Hinduism shifts to the Ganges Region, as the Indo-Aryans migrate there, with Mahabharata and Ramayana being formed there.
The Indo-Greek's capital under Menander was Sakala (Sialkot) and regional capitals such as Taxila and Charsadda existed. Alexander's campaign in 'India' took place entirely in the Indus from his Pyrrhic victory at the Hydaspes (Jhelum) against Raja Pauros too getting almost mortally wounded at the Battle of Multan. Again, having the very North-Westernmost parts of modern day India does not change anything. Look at the map:
Almost the entirety of modern-day Punjab, beyond the Indus, was under the Achaemenid so while it indeed could be called a frontier region, it was by no means a boundary.
Not to mention the Parthians (Pahlavas) too ruled the entirety of the Indus Region, almost mimicking the boundaries of modern day Pakistan, with almost no modern day Indian territory in it.
Even Adi Shankara, an 8th century Hindu saint, marked the four points (mathas) of India he added Dwarka, Gujarat as the westernmost point, Uttarakhand as the northernmost, somewhere in Bengal as the easternmost and a city in Tamil Nadu as the southernmost. Again, none of these territories are in present day Pakistan
Coming to linguistics, only 3-4% of Indians
speak North-Western Indo-Aryan languages (Sindhi, Punjabi and Dogri) while they are spoken by a majority of Pakistanis. Dardic languages arent spoken in India and neither are Iranian (Pashto and Balochi) languages spoken. Urdu is a mix between Central Indo-Aryan and Iranian and anyway its the native tongue of only 6% of Pakistanis. A minority of Indians speak Hindi (which all linguists classify as being seperate to Urdu) as a first language. Infact, in South India many dont even understand it, let alone speak it.
Please go Google the Harappa DNA Project spreadsheet, a genealogical study by an independent American genetic institute. I can't post the link since it's not allowed (dont know why). It shows the different genetics of Pakistanis and Indians. Pakistanis (Punjabis, Pashtuns, Sindhis and Balochs) have much lower South Indian component and a majority Baloch (which means Gedrosian Iranic, nothing to do with actual Baloch ancestry) component, while Indians (UPites, Bengalis, Biharis, Gujratis, Rajasthanis, Tamil, Malayalis etc.) have higher South Indian component and lower Baloch/Gedrosian Iranic component.
All this goes on to show that we are ethnically, linguistically, historically and culturally two different people.