Hi! As Clark said, I would extremely recommend adopting a proper authentication system. What you're proposing is extremely, extremely insecure, and could lead to social and legal ramifications in the event of data breaches (which are near guarunteed with this system). If you adopt this, you should reasonably expect your own admin account to be cracked within hours of launching the site. Is there a reason why you are so strongly against using a sensible login system?
As to your questions about Flarum:
maulik-neosoft Is this good idea to use Flarum instead of using owned custom setup from stretch in PHP?
Developing a good forum system is hard. Really hard. Flarum is actively developed, has an extensive extension and support ecosystem, and is actually somewhat accelerating in terms of progress. If you're looking for a modern forum system that your users will actually enjoy using, Flarum is your best bet.
maulik-neosoft How easy to customize or create own extension or to override core features like login and registration?
Flarum is extremely extensible. In the frontend, you can override literally anything; in the backend, there are extension mechanisms for practically anything that could sensibly be extended.
maulik-neosoft I also wanted to customize entire admin side so I can add users from admin side itself.
You can definitely do this. There's actually already an extension for this: https://discuss.flarum.org/d/22608-create-user-modal