Normandy and Aquitaine were historically linked to the English crown, even if the latter at least paid homage to French sovereignty. So, these areas have land borders with the rest of France. English nobles bore titles that related to Normandy, especially, such as Count of Eu (the Bourchiers). Brittany is also not integral to France in this period, has historic links to England, and in a later period the English would campaign there in Elizabethan times.
I could easily see that the Angevin/Plantagenet homelands are appropriated by the Crown, and dished out to loyal nobles - this would push an English zone further.
One of the main problems that any long-term King of England-France will have is that of overly powerful nobles, such as the Dukes of Bourbon (unless they are attainted)
By the time of the 100YW Normandy was lost by the English kings for few centuries so no land border there and while Gyenne was still loyal supremacy of the kings of France had been recognized even there as had been demonstrated by Charles V.
Brittany was, indeed, still just a vassal of France but fighting between French and English candidates was a prolonged affair and the Duchy’s interes did not include helping anybody because strengthening of any side would be dangerous for its autonomy.
Could the English conquer France (BTW, why is there an implication that by default everybody should be sympathetic to the English side)? I seriously doubt it by a number of reasons:
1. Even with the superior military system and extremely incompetent leadership on the other side this did not happen and when there was at least some competence on the French side, they were losing conquered territories.
2. England being a much smaller and poorer country than France, a successful conquest would need a serious local support but it started with a wholesale looting and continued this way for the next century. Of course, there were some feudals who were taking the English side but their loyalty was quite unreliable as had been demonstrated by Burgundy on the last stage of a war. Was it possible to attract enough of the leading French aristocrats to the English side to guarantee a political victory? If it was, Valois would not be elected a king Salic Law or not. Even in the best (for English) case scenario when the Burgundians and Orleans were at each other throats, the king was insane, Paris was on Burgundian side, and English controlled most of the Northern France opposition proved to be too strong.
3. Could he English conquer France by a pure intimidation? Highly questionable because their armies were too small for the task, their military system had serious limitations and even their best commanders were just good tacticians. After victory at Poitiers Esward’s Reims Campaign was a failure: he could not take Reims and he did not even seriously try to take Paris. The only thing that he and his best commanders could do was to waste a countryside in a hope that an enemy would be stupid enough to stick out his neck for a field battle. BTW, and this is a pure curiosity, were the vicinities of Chartres completely lacking villages, farms, etc. or was a need to make a camp on the “open plain” a byproduct of the usual tactics of destroying everything on the route? Surely, the soldiers placed in the houses would not suffer from a hailstorm. Anyway, on the later stages of the war the chevauchee tactics was routinely failing in its goals.
4. Could the English get a wide popular support? Unlikely, both because looting and destruction are not the good tools and because they proved to be rather inept administrators: Ed Jr. managed to get Gyenne rebellious (notwithstanding the traditional ties) by his administrative methods. Could this be changed? Unlikely, because England could not finance war forever and with a passage of time the prevails no opinion was that it should be paid for by the occupied French territories. Rather difficult to get love of the people whom you are looting. The only thing they accomplished was a raise of the French nationalism.
5. Was English military system superior enough was the task of conquest? Unlikely. Of course they did have serious tactical advantages over the feudal militia but their main strength was in a defensive battle in a prepared position. When the battle was denied, they could do little besides looting and a skilled opponent could keep squeezing them out the conquered territory as Du Guescklin was doing. When they were attacked before they could choose a good position, they were defeated as at Patay and with an appearance of the field artillery and firearms their tactical advantage was gone and they had been defeated at Formigny and Castillon.