Hey Noods, from my understanding, calcium carbonates (90% egg-shell content) are more thermodynamically stable as calcium oxides (caustic lime). Hence heating the egg shells in excess of 380K (107C) will lead to oxidation of the carbonate, to form calcium oxide where the carbon moiety will be liberated as carbon-dioxide. Heating will also serve to denature the trace protein complex (5%). Now calcium, in it's carbonate form will serve to neutralize soil acidity as and when it arises. The oxidation state of calcium in it's caustic form however, is not as effective as the pulverized carbonate limestone for horticultural pH control and management of soil-cultures. So my take on this is to apply the lime in it's carbonate forms and refrain from flaming it.
Since carbonates will register in the EC, their contribution should be ignored from the salt content indicated by the EC measurements. This is why it is sometimes recommended to water-culture growers to subtract the contribution of carbonates to the EC since this factor is magnified under aquatic conditions.