कष्टाचे फळ

It's better for significant efforts. For small things, just say 'छान झाले' (It turned out well).

'Kashta' implies more physical or mental struggle than 'Mehanat', but they are often used interchangeably.

Usually, yes, as it represents the 'concept' of reward. But you can use 'Phale' (plural) for multiple successes.

No, it is almost exclusively positive. If work leads to a bad result, we use 'वाईट परिणाम' (bad result).

You say 'तुला कष्टाचे फळ मिळेल' (Tula kashtache phal milel).

Yes, it's very common in business to describe profits or growth.

There isn't a direct single idiom, but 'फुकटचे यश' (free/unearned success) is a close conceptual opposite.

Not at all. It is a timeless classic used by all age groups.

Yes, it's a very polite way to congratulate someone.

Yes, it's the retroflex ळ. Practice by curling your tongue back to touch the roof of your mouth.

Yes, it's perfect for a professional but warm congratulatory email.

Culturally, yes. It reinforces the idea that every action has a consequence.