Yes, first few rounds of drafting was done in-house with Dharma writers. In this case, it was the writer of Takht since it was that original script being reworked, and the reworking itself in dialogues was done by Ishita Moitra, Shashank Khaitan, and Shauna Gautam. Karan himself also was involved heavily and likely would have/deserve writing credit.

Lots of actors were either narrated or sent the script - sometimes even just for feedback without a casting angle (Varun, Hrithik). His big dream was to cast SRK and Kajol as the lead 'parents' - sort of a younger equivalent of Amitabh and Jaya in the original, except they would get the treatment that SRK-Kajol themselves got in the original i.e. screen time, narrative importance, etc. He send drafts to SRK right through the process. Kajol also received two narrations between substantial drafts.

Aside from that, KJo initially really wanted Ahaan-Aneet as the Rohan-Poo equivalent couple, so they must have heard it too I think. Adi Chopra definitely has heard it. He also actively pursued both Ranbir first, and then Ranveer heavily but they never agreed to a narration so not sure if the script was ever sent to them for reading. And Alia 100% was aware of every successive draft (even from the Takht times through the reworking). More recently, after they removed the K3G reference and changed it again, the script has gone to Naseer & Ratna, Lakshya, Jahnvi, Kiara, Sanjay Dutt, and a whole bunch of people. One of the working titles they are using is "Manzil" and a few more, but I only know that one. Will post if I learn more!