The change to input 4 of the emonTx is the burden resistor - this determines the voltage that the c.t’s secondary current generates. It’s the resistor immediately behind the jack socket, and in the case of input 4, it is 120 Ω (22 Ω for the others). There are two holes either end of it for a wire-ended component, so you need to add a parallel resistor to bring the total resistance down to 22 Ω - 27Ω is very close. A 0.5%, 0.1 W would be my preference, but you might need to settle for 1%, and 0.25 or 0.33 W won’t hurt provided the body is short enough.
You’ll also need to change the calibration constant in the sketch, so if you don’t have one, a FTDI programmer needs to go on your shopping list.
Yes it is, you can add the ESP8266 WiFi Adapter for EmonTx to the serial output. Have a read of GitHub - openenergymonitor/EmonESP: ESP8266 WIFI serial to emoncms link
The problem with
is, we don’t have a two-channel receiver for the Raspberry Pi, so WiFi is safer. But the r.f. collisions problem depends of course on how many transmitters you have. The risk doesn’t become serious until you have more than a handful.