Sillo,
Couple of things here, the calorific values of fuels varies by tiny amounts, not worth the discussion point. The different octane ratings means you get to use the fuel with higher boost or higher compression ratio to achieve more power, while maintaining detonation margins that are acceptable. And acceptable varies from airplane to race car and everything in between.
As for if an EGT probe fails? Well no, once you are "known to be LOP" a probe failure matters not, because validation from all the others, your fuel low etc…..means the EGT probe is just validation of a know state.
You can comply with the POH supplement without even looking at the egt info if you want.
