To see what've involved in combining documents with different page layouts, see my post of 1 Oct 2017 at:
http://www.eileenslounge.com/viewtopic.php?f=26&t=27951, particularly the 'LayoutTransfer' sub there. On top of that, you have the content formatting issues you've already identified to deal with. They arise primarily because of Style conflicts, which will only be exacerbated if people have been overriding the Style definitions with direct formatting.