Airtel giving CGNAT: True.
Does it have anything to do with bridge?: No.
Coming to your question:
1. Cloning
MAC will work as long as BSNL uses MAC binding.
2. When you set your
ONT to bridge mode:
ONT becomes a basic fiber to ethernet convertor.
You let firewall and everything to the
router which you trust will be secure and handle the internet for you.
ONT lacks features and Router offers more features (Depending on The Price). People prefer letting router do everything coz ONT performance is not that great and it lacks way too many features.
This is the reason I mentioned to look for Openwrt compatible router coz then you'll have a custom firmware which gets as many feature as your Router CPU and parts can handle.
Openwrt is basically unlocking bootloader and flashing the latest
Android OS on your phone where your manufacturer has stopped providing updates for your device.
3. Its the same thing. You decide which one out of the two (ONT OR ROUTER) you wanna allow exposing your IP to the internet.
4. Your router handles DHCP when you put ONT in bridge mode.
When you use ONT in route mode and connect LAN, DHCP will lease a private IP to your PC connected via LAN. Same thing router does.
Note: If you change the MAC address of your PC to some other device on your network then only it will lead to a MAC conflict. Since all devices have their own unique MAC addresses, you don't see MAC conflicts as long as you don't mess with them.