1 I'm not an immigration expert but your tax domicile won't be affected by your being given indefinite leave to remain. You will remain as domiciled In India, at least for now, so if you left your share of the gain out of the UK, you could choose to be taxed on the remittance basis which you can read about in section 9 here
. Buying a property in the UK makes no difference, at least not now.
2 If the gain is taxable in India, the UK/India double tax treaty will ensure that the tax paid on the gain in India will be deductible from the UK CGT liability. If there is a shortfall, then you will need to pay the balance.
3a You will be taxed in the UK on the basis of your ownership share.
3b You use the exchange rates at the time of purchase and the time of disposal. You can use the spot rates on the relevant days or the average rates for the UK financial year which ends on 31 March. Look here
for exchange rates.
3c The calculations are correct. The rate of CGT that you will pay, 18%, 28% or a combination of the two, is dependent on the level of your income in the tax year of disposal.
I hope this helps but let me know if you have any further questions.