Is it a mainstream belief among Indian academics that Jews (descendants of ancient Hebrews) really came from Indian Brahmans?

"academics" can not possibly hold such position.

The notion is a ludicrous, baseless one that has no place in academia.

That some Greek historian would use the same word for Jews as for Indian means nothing with respect to the origins of the two groups as Greeks first encountered Jewish culture at the time of Alexander

(and encountered Jews unknowingly in the Persian armies at Thermopylae)

If the first impression of Jews was as one of the exotic peoples of the Persian empire, it is reasonable that they would merge the two.

There is the theory that some ancient Israelites could have traveled to India and their descendants then became the Indian Brahmins

Naturally Indians would like to believe the opposite is true, but it's far more likely that the Jews gave rise to the Brahmins, not the other way around.

Jews are said to have resided in Afghanistan for nearly 1,500 years, since Afghanistan is close to Northern India it's not a stretch to believe that Jews traveled from there to Northern India for reasons of trade.

Jews were also in Babylon (Iran and Iraq) during the time of King Cyrus (500 BC). It's not very far to travel from there to India since Pakistan was part of India at the time and Pakistan is right on the border.

Never heard of that, and indians have far-fecthed theories which nobody takes seriously.

These idiots believe an elephant with 5 arms swallowed the moon and earth. What a bunch of mofos!

Not a main stream belief

Yes it's true