I could be entering the realms of diminishing returns
Indeed.
One thing I would do is seriously look again at a filtered master socket faceplate, separating the ADSL and voice signals at the optimum point.
Obviously I don't know your house, but remember when placing a wireless modem/router:
1. It only uses 12V DC power; you can easily extend the 2-core power lead yourself (just don't reverse the polarity!).
2. You can have almost any length of cable from the filtered faceplate to the router, just use the correct spec STP type as sold, for example, by ADSLnation.
3. You can install wireless repeaters if coverage isn't ideal and some of these also have LAN connections
4. Powerline type adapters can also be used to extend the network reach
Something to consider and I think would allow you to do it suitably tidily.