Here is what is probably the authoritative work on the Salic Law. It does not seem to agree with you.
I think you are confusing attornment (which was part of the mediaeval succession process) .with the modern idea of election. When the men of the middle ages spoke of 'election' they meant it in the sense that theologians do - as in " Neither are any other redeemed by Christ, effectually called, justified, adopted, sanctified, and saved, but the elect only." (Westminster Confessional). Not what modern folk mean by the word, someone being voted into power. God elected Princes. Not men.
The practice of "electing" the sons of Kings in their fathers lifetime was common. It was a role copy of the King of the Romans/ Holy Roman Emperor paradigm. "If the Emperor has his vassals recognise his son as his successor , maybe I should do the same". Difference was, that the HRE really /was/ elective.
I don't think this thread is about Edw III becoming King of France. It is about retaining the Angevin possession in the area now known as northern France.
My understanding concurs with Matteo's. For much of the High Middle Ages, the King of France was elected by an assembly of nobles.
As much as I can piece it together, the practice of election seems to have started late in the Carolingian period, as a way of bypassing an incompetent heir or resolving an ambiguous succession. Both France and the HRE carried on the practice. The divergence in actual practice and France's transition to a strict hereditary monarchy seems to come down to a few major factors:
- The Capetian dynasty had much better "dynastic luck" than their counterparts in the HRE, specifically a long series of competent kings, each of whom had a male heir and lived long enough to arrange for that heir to be elected during the father's lifetime. In the HRE, there were regularly opportunities for the electors to exercise real power rather than just ceremonially confirming the King's geneological heir.
- The Imperial title itself was a bit of a handicap, especially because of the precedent that the title was in the gift of the Pope. French kings had their sons elected as junior co-monarchs (sharing the title of "King of France"), while neither the electors nor the Emperor had the power to create a co-Emperor without the Pope's approval (when a successor was pre-elected, he was usually elected to the lesser title of "King of the Romans" or "King of Germany"). The requirement for Papal approval also gave the Pope the ability to upset and complicate succession plans.
- France was simply easier to rule, by virtue of being geographically smaller and more culturally and institutionally homogenous.