1) Can retail investors buy GOI bonds through zerodha account?

Zerodha
1
2

Investors can Only buy Gold Bonds issued by Government of India like SOVEREIGN GOLD BONDS and if the bond is listed on the secondary market, then invester can buy it.

REC Bonds also from GOI i guess,

If the bonds listed on the exchange then you can trade.

3

Hi,

This question has been answered here.