This issue is similar to what happened with the Volkswagen emission cheat. After they fix the ECU to have the advertised NOx emissions, the car lost peak power and fuel efficiency.
Powerline Ethernet might be a solution for the cable problem. I'm thinking of connecting the extra AP(s) through Powerline Ethernet to the router if there is no copper Ethernet available.
This might be the wrong thread to recommend it, but I've had good luck with TP-Link's Powerline Ethernet adapters. I switched to them from WiFi for my living room TV/Gaming/Streaming setup. I'm in a high interference wireless environment due to neighbor density (even in 5Ghz) and the TP-Link devices reliably push over 500mbps.
A downside of powerline ethernet devices is they use your house wiring as a hub-style network. While I think some support a form of MIMO, the more devices you have the worse performance will be. The TP-Links I have sport QoS, but I've not messed with it.
That's exactly what I did and it's worked out great. I'm getting upwards of 300Mb/s using powerline adapters. They were netgear brand, I apologize I don't remember the model, but I think it had "1200" in it. A lot will depend on what else you have on the breaker, from what I understand. But for my purposes the performance was more than adequate.
Powerline Ethernet is not really a good solution either. If you don't use your phone jacks, take the cover off one and check if it's actually using Cat5 or greater. You can then use those for network if you replace the connectors.
You really shouldn't make blanket statements like that. While I'd definitely recommend using existing category 5 wiring if it's available, I'm getting close to 300Mb/s via powerline adapters, so it really comes down to your individual situation. For me it's been more than adequate.
As a no longer suffering user of X10 now enjoying ZWave, the issue is usually crossing the out of phase 110 legs (every other full size breaker in the panel is on a different leg).
To cross the phases, the signal has to go to the power pole where the phases originate. X10 has a repeater you can install and sometimes people just install a passive one (a capacitor if I recall.)
In a smaller house, signalling to the panel and back out to circuits on the same leg is more likely to work than crossing the phases.