Capital gain purchase date

Under construction Property purchased by Party A from builder but due to financial issue builder transfer that property to Party B and party B paid to party A 53lakh in 13 june 2013 and 7lakh to bulder in 4 years and party B also paid interest for 10years amt 23lakh from bank loan also paid stamp duty amt 2.5lakh on 9.9.2021 which registration date. Builder given possession letter date 21.11.2020.
1)My query is what will be date of acquisition
2) Interest amount will be add in purchase property amt
3) 24b calim for 3year then after not claim then rest year can I add in purchase property amt
4)tax regime 20% and 12.5 anyone can I choose?