Please teachers, I am not sure if I can use 'whether' this way.

Yes, both are fine.

Thank you.

I would like one more sentence checked, as below:

- Whether to have to fight against him or not, I feel prepared for whatever coming.

Is this sentence correct grammatically and understandable?

It's understandable, but not quite grammatical/fluent:

Whether I have to fight (against) him or not, I feel prepared for whatever is coming.

Thank you.

Cheers. Hope it helps.

Regardless if I was asked or not (about it), I will tell him the story anyway.
Be careful using 'it', unless we know what 'it' is.

Thank you.

But I wonder if 'Regardless if' is the same as 'Regardless of whether or not', and can the two be used interchangeably?

- Regardless of whether or not I was asked, I will tell him anyway.

Is this ok and still meaning the same?