Lemom and Lime

Limes are sweeter. They are so similar but I agree with SCP - they have a completely different flavor profile. Mexican foods (and cocktails!) call for limes. You would never squeeze a lemon on a grilled fish taco. Mediterranean foods call for lemons. You can't sub limes for chicken with olives and lemons. Each is just wonderful in a salad dressing but they are different dressings.

Depends upon whether you're referring to the fruits from an American or Indian perspective.

The small fruits that are available in that are called 'lemons' in Idia are called Key Limes in the US of A.and the larger fruits are referred to as Limes.

Lemons in the US have a thick rind, a bumpy texture & are bright yellow. I don't believe those are readily available in India. Kinda reminescent of 'Naarthangai'
They have a different aroma than the limes (Indian Lemons).

I think it all depends on what you are making. In my opinion, they have completely different flavor profiles and, in terms of taste, are not always interchangable (for example I would sub grapefruit juice for lime and would not use lemon in Thai curry recipes). If you are just looking for acid, then they can be exchanged (such as a salad dressing). Lemon is a classic seasoning and can be added to brighten just about any French dish close to the end, but I'm not sure I'd use lime, it just doesn't have the right flavor. Mexican black beans? Lime would be my choice, but I think lemon could also be very good. Mostly, just a long way of saying...it depends.

I would say in almost all cases, yes. I think of lime juice as a little sharper with a bit more sting. Limes are also a touch smaller; so if the measurement is "juice of one lemon" you might want to add a bit.