Increase the power transmitted by the emonTH, at the expense of battery life.
Use a directional antenna, at either or both ends of the link. This is only practical for the emonPi, and then only if all the sensing nodes that report by radio are in the same direction from the emonPi.
Improve the efficiency of the standard antenna(s) with a ground plane (search for a long thread about this - you participated in it).
No such thing exists. All you can do is trade the power radiated in one direction against power not radiated in other directions. A bit like the reflector behind a light bulb. That works when receiving too.
It cannot affect the data sent; but it will affect the strength of the received signal. If you don’t have an antenna on the emonPi, you’re unlikely to receive the signal from the emonTH, unless it is very close.
It cannot affect the data sent; and normally, it will not affect the signal transmitted. The two frequencies are vastly different - Wi-Fi operates at around 2.4 GHz, the ISM band radio in the emonPi at 433 MHz.