Effectiveness is influenced by capacity of unit. If you are trying to mount a 270 cfm fan more than 30" above cooktop, then it may make noise but will do little to clear the air. If it is 1000 cfm, then it can be higher than 30". So the answer is "it depends". Most people don't mount them more than 36" above the cooktop unless it is a really high powered unit.

What a beautiful house, Shahbaaz! I live in a similar house - a 150-year-old colonial bungalow in Kumaon. You have already received lots of excellent advice from better-qualified people, so all I would say is, keep its age and heritage character the central theme. The white tiles do clash, I am afraid ... flooring with terracotta tiles would echo the roof and look lovely. The columns and the arches over the doors and windows are amazing. But I'd say, leave it all chuna-painted - there's a beauty and softness to it. Don't let the painters add too much neel (indigo) in the limewash -- in the hills where I live, they put far too much if I don't watch out and the result is a harsh white. Natural lime has a softer, creamy look.

that is what i have added if you can see the pic above.. want some suggestions on partition that i can use between dining and sitting

The distance between chimney and hob needs to be 26"-30" : 65cm-80cm. placing it above the window, suction power is reduced, placing it in middle of the window, will make the window ineffective. Island chimneys are best suited to be positioned in middle of the room, the duct running through false ceiling. also they fall in the expensive category. it is best that you move the hob position to side of the window, or totally to the other side and do chimney core cutting there.