On shipping and canon consistency

This ramble was inspired by one of After the Rain's posts on canon and non-canon shipping -- I intended to post a reply, but then it started to turn into some kind of mini-essay, so I reckoned I'd better post it here instead. So here's my 2p's worth (or 2d's worth, I suppose, when that phrase was coined!)

( Why it's possible to ignore the blueprints used by Rowling Shipwrights Inc and still build a reasonably seaworthy vessel )