Yes, this could work, as the timber framing should be a lighter weight. Subject to a Structural Engineer confirming the loads and foundations etc. are adequate. Flat roofs cut into a pitched roof may not be acceptable under Planning Permission though. If existing foundations not strong enough you can consider reinforcing / underpinning the foundations with concrete or galvanised steel screw piles such as Chance piles. http://www.subsurfacetechnologies.com/chance-foundation-systems/underpinning-helical-piles. Underpinning will add on considerable cost but means you could continue with traditional masonry walls on top of the existing walls. Hope this helps.

Jonathan is absolutely right and on this one I would suggest that you ask your Architect to speak to the contractor to give them their rationale. It also depends upon the type of pipework that the contractor proposes for his alternative location.

I too suggest painting it. Makes such a difference. You’d need to get in touch with a decorator/painter for a quote as it varies considerably depending on where you live. It is a fiddly job and some painters don’t undertake this kind of work. Some on the other hand specialise in this so ask around for quotes.

I recently just constructed my very own flat, when i was researching I cam across Build In Digital, where I signed up to share my own construction story. You get the chance to connect with other people in a similar situation, this might be something you might like? Sorry not very useful for answering your question!