Temperature inaccuracies when using PT100 sensors are usually caused by poor connections. Each ohm of resistance in the connections or the cable will increase the reading by about 2.5C. I have this problem on one of my owm printers at present, I can make the reading change by more than 10C by waggling the cable close to one of the connectors.
Hmm, well I swapped with my other PT100 sensor and printing was transformed back to normal? I ordered another one tonight. Once I have 3 I may mess around with them.
Never mind, i figured it out. The reprap firmware configuration assistant was using M208 to set the minimum z height to 1mm. I changed it to 0mm and now when i run the calibration I'm getting 0.1mm as expected.
To be honest, the configuration assistant was more trouble than help. I'd almost recommend not suggesting it in your documentation.
That does sound like a problem with the WiFi module. It's possible in theory to set the Mac address in firmware, but we don't use that facility because each WiFi module is supposed to have a unique default address.