Kusten, the inlet pipe is usually fitted under the floor, when supplying an island hob. Or if the hob is close to an outside wall then through the wall directly behind the hob. I find it works best,when the air can rise all round the hob. It is not linked to the extractor, if you have a very powerful extractor, and therefore need a lot of make-up air, then inserting a fan in the pipe, a fan with the same capacity in cubic feet will solve the problem. As normally fitted, extractor fans, without make-up air provision, pull outside air from wherever they can get it. Mainly badly fitting doors and windows. This causes drafts and means you are loosing your expensively warmed air blown from the room and replacing it with cold outside air, which you then have to heat. The extractor has to pull quite hard to get the large volume of cold air it requires from outside through a narrow crack or gap round a badly fitting door. make-up air arriving through a six inch diameter pipe has a much easier path and that's the way it will arrive. You will instantly notice the difference, your kitchen will be warmer and your heating bills lower. So called extractor fans that don't exhaust to the outside are a joke. All they do is filter out grease in a block of carbon. All the water vapor that is produced by burning gas and cooking remains in your home. The first law of thermodynamics is that heat always moves to cold. Pushing hot wet air into an unlined chimney or cavity is asking for trouble. As the water vapor instantly condenses on the first cold surface it meets. A cold wet wall/chimney provides an express route outwards for your expensive heat. All modern insulation works by trapping air in small cavities, air in a space smaller than 19mm does not allow rotation and therefore does not pass heat by circulation. A damp chimney quickly and permanently passes heat from the room into the chimney and up into the sky. You don't want this to happen.

Perhaps you could use the mantle elsewhere - even faux fireplaces can make for a nice focal point if you need a feature in your newly repositioned living room? I also think the way a house 'flows' is one of the most important elements of how well a space 'fits' it's owner. You've got to make it work for you and the way you like to live, if that means switching some of the rooms round then absolutely go ahead. Especially if it's going to be a long term or forever home.

We live in a house which was originally two small cottages both 100yrs old and are currently saving to achieve the same as yourself. What I've found helpful is to measure out things like a sofa and dining table onto tracing paper and you can shift things around on your plan to see how it would function in real life if that makes sense. We have renovated the house in 3 stages so far with more to go. To be honest it's been a steep learning curve and the initial plans the architect gave us would have put the house into negative equity. Just get yourself a scale ruler and you can play about with the design yourself. Hope this helps.

Be mindfully of the ducting you use, some can be  very noise if not secured and echoes in the kitchen.but good value for money, equivalent AEG or Bosh can cost 3 times more