Was either side legally in the right in the Hundred Years War?

The legal situation was not as clear as the question assumes, because neither of the reasons cited were valid at the time. While people often apply Salic Law to the dispute in 1328, this is ahistorical - Salic Law had long been defunct by then. Royal succession was not fixed in legislation, but instead shaped by customs that had evolved over the centuries.

[The Lex Salica] was slowly forgotten until the middle of the fourteenth century, when it was rediscovered by Richard Lescot and the monks of Saint-Denis. The Salic Law enjoyed a second lease on life from the beginning of the fifteenth century, when it was officially adopted by the French Crown as the post-factum justification for the exclusion of women from the royal succession.

_{- Taylor, Craig. "The Salic Law, French Queenship and the Defence of Women in the Late Middle Ages." French Historical Studies 29.4 (2006): 543-564.}

Though it was not called 'Salic law' at the time, François de Meyronnes had already written a treatise on the lex voconia, which excluded women from succession, enshrining the fundamental principles adopted later in the century to justify royal practice in France.

_{- Jones, Michael. "The Last Capetians and the Early Valois Kings, 1314-1364." The New Cambridge Medieval History. Vol. 6. Cambridge: Cambridge University Press, 2000. 394.}

France's succession disputes of 1316 and 1328 were essentially constitutional crises which had no precedents, since the Capetians had not ran out of dynasts before. The precedent of excluding women was first established when French magnates chose Philip the Tall over Joan II of Navarre in 1316, and extended when Philip the Fortunate was chosen over Edward II of England in 1328.

Both choices were motivated by compelling, political reasons. Joan was only four, and of questioned paternity; Edward was English, and the it was not altogether obvious which of the last Capetian princesses possessed (and imparted to their sons) a better claim, were female succession to be accepted. The succession was thus determined by the interests of politics, not legality.

The murky legal situation is why both female and male claimants (or their representatives) met at Saint-Germain-en Laye to settle the dispute with compromises and concessions

It is important to note that the so-called "Salic Law" was only cited much later; the first known reference to it in regard to the succession of females was made in 1358 ... Although later writers, such as François Hotman, claimed that the Salic Law had been invoked in the succession crisis of 1316-28, there is no evidence from the period to support this claim. As Éliane Viennot argued, "No law, principle or judicial theory was advanced. Only the rule of force and interests were brought into play."

_{- Woodacre, Elena. The Queens Regnant of Navarre: Succession, Politics, and Partnership, 1274-1512. Palgrave Macmillan, 2013.}

Matters came to a head when Edward III's fief of Aquitaine was confiscated. In the ensuing war, Edward advanced his claim on the French throne. Out of political considerations, it became expedient for the French bolster the legitimacy of the House of Valois over bypassed claimants from female lines. Salic Law, including the prohibition of transmission through the female line, was "rediscovered" to do just that.

[A]s Taylor notes, "the problems that French intellectuals faced as they tried to deal with the precedent set by the exclusion of women from the French royal succession." This precedent needed considerable justification, not necessarily in terms of a misogynistic need to argue for the inability of women to rule effectively but a very practical need to confirm the legitimacy of the Valois dynasty against the claims of Edward III of England and Juana's own son Carlos II of Navarre, who both had very strong rights to the French crown, through the female line.

_{- Woodacre, Elena. The Queens Regnant of Navarre: Succession, Politics, and Partnership, 1274-1512. Palgrave Macmillan, 2013.}