by then

“When Raman returns from office, Sita would have finished the cooking by then.”
Is this sentence correct?
Or can it be rephrased in a better way?

By the time Raman returns from the office, Sita will have finished cooking.

Can I say as below:
Raman returns from the office, Sita will have finished cooking by then.

That doesn’t work because of the tense mix.
You can say:
Raman will return from the office. Sita will have finished cooking by then.
Raman returns from the office. Sita finishes cooking.

However, they are not as natural sounding as the single sentence in message #2

Beeesneees,
“When Raman will return from the office, Sita will have finished cooking by then.”
I believe this sentence is now OK.
Am I right?

No, it sounds really odd using ‘will return’ with ‘when’.

Beeesneees,
What about this sentence?
“When Raman returns from office, Sita will have finished cooking by then.”
Is not correct?

You don’t need ‘by then’ and it sounds strange if you add this redundancy.
This is okay (don’t forget the article with ‘office’):
When Raman returns from the office, Sita will have finished cooking.