Oops, I didn't notice your paragraph "After a little while I noticed a really acrid, burning-electrical-insulation-type smell, which seemed to be coming from the MagSafe connector itself. This seems to have become ever so slightly discoloured, so I put it down to a short in the connector itself, and discontinued using it."
So clearly something was burnt out.
Nevertheless, would replacing the I/O board also solve the problem (at considerably higher cost) if dirt were present and the cause of a poor contact -- is a new female MagSafe connector present on the replacement I/O board?