You can share network ports, after a fashion, so long as you don’t need Gigabit speeds. 10/100 Ethernet only uses four wires in the 8-wire cable, so with a bit of hackery it’s possible to shove a second connection down the other four wires, then a similar hack at the other end to separate them out again.
However, for that to work it’d need to be a passive connection between the two ends, so running it over something like Homeplugs will be a non-starter. If you’re only ever going to use one device at once and you’re after a ghetto solution then the only thing I can think is to put the Homeplug somewhere accessible and unplug / plug in whichever cable you require at the time.
But TBH, you had the answer in the first two posts. Either a multi-port Homeplug or a four-port switch (or ‘number of devices’ + 1 really). £6 on jimdubleyou’s link, and from a technical standpoint it shouldn’t need any configuration at all beyond plugging it all in. If anything, it’ll be cheaper and considerably simpler than any cable-splitting tomfoolery.