Hi
1. Not many cons it is an effective why of insulating the room the roof is over. However it raised the roof height by about 120mm so if there are any low windows or balcony doors on to the roof it can be a problem
2. I’ve not had one on my home yet but I’ve been told by customers it makes a massive difference
3. The osb or ply at the base level should always been 18mm, 120 is the common thickness of insulating boards and depending on your membrane I prefer torch on felt. Felt can be applied straight into an purposefully made insulation board.
4. I use Hyload self adhesive vapour check but I’ve been on MOD sites where the spec was literally a sheet of polythene so I think they all work well
5. I’m not really sure what you mean but the boards are totally sealed yes. You shouldn’t be able to smell them
6. Sky lights and lanterns are a great way of bringing light into a room. The curb is made up of 4x2 timber that the light sites on. There are so many places to buy them from. If you are stuck go to a local roofing merchants and they will point you in the best direction

Hope this helps!

hi
as the answer above is a true statement , adding to that you want a breathable membrane ,

you can get lanterns from eurocell , suntrade or stevenwoods who I use

also your going to want firings on the roof to divert the rain water otherwise the water will start pooling on your roof

I recommend GRP fibreglass roof it last a lot longer

hope this helps