@Danal
The heater erratic behavior is on the configurator page only at https://configtool.reprapfirmware.org/Mapping
It's perfectly fine on the printer itself! I actually managed to finally get a calibration cube out of it 
Thanks for confirming the case of M500.. Yeah 
I ended up doing the M307 for H0 and H1 by hand and filling them up there and then doing the M665 and M666 as well. (I'll move them to the config.g since they don't really need to live in the override either.)
So far so good!
I'm all new to RepRapFirmware and coming from old Repetier 8 bit world. 