1. Yes, capital gain will still arise as for the purpose of calculation of capital gain, the sales consideration will be considered as higher of actual sales price or circle rate.
Further, the difference of circle rate (17500 per sqft) and selling price (4500 per sqft) shall also be taxable in the hands of buyer under section 56.
2. Yes.
3. Yes.
4. Deemed buying price of y will be INR 17500 as Y has already paid tax on the differential.
5. There will be a host of case laws on this topic and it will be difficult to reproduce the same.
6. Sorry I didn't get the ask here. Request you to take a phone consultation for detailed discussion.