Is it true France never fully recouped itself after the Great War (despite winning it) and simply wasn't ready for WWII?
France was materially quite ready for WW2. Their failure was tactic and political. And it they didn't had so many fools (Gamelin) and traitors (Petain) among their generals they might have even recovered and made the germans bleed so much that the whole encirclement at Dunkirk thing was turned around. It was a close thing. It was one of those cases where erratic behaviour was worse than just letting the original strategy, though flawed, be carried on: Gamelin was correcting for the original failures when he was replaced, Weygard failed to do so, Petain was a phony...
Was Germany's surrender unavoidable?
The Central Empires were weaker in terms of resources, due to the british naval blockade. But had done a good job during the first half of the war of seizing alternative sources of supply. Some blunders like having Romania become a belligerent, losing and being quickly occupied, very much helpers the Central Powers.
My opinion is that until 1917, despite the imbalance in resources, it could have gone either way: a peace favorable to the central empires (but with gains essentially to the east), or their defeat as it happened. The collapse of Russia created an opportunity for them to win: France and Italy were hard pressed defending their borders and could be really threatened if the germans and austrians made good use of the troops released from the eastern front after the separate peace with Russia. And the british were not faring that well against the Ottomans yet.
But they wasted it. First by adding the US to the ranks of the Allies for no good reason. Germany by 1917 still lacked a submarine fleet capable of choking the UK, and the two years of respite it had given the british since 1915 had allowed them do develop countermeasures (depth charges and the hydrophone). And then by splitting their troops across several offensives along the western fronts, instead of hitting , and hitting hard, on one place. Italy might have been knocked out of the war - but the germans pulled out their troops from that front after Caporetto, and then the austrians were incapable of success at any further major offensive, and finally disintegrated an Vittorio Venneto....
In Greece a (comparative) handful of french and british troops, together with the remnants of the serbians, kept pinning central power troops and threatening both communications with the ottomans and vital trade in scarce resources from the balkans - they could and should have been pushed to the sea: evem the ottomans were capable enough to do that in the Dardanelles! But no, instead of hitting the aliies on their weakest points, the germans just had to keep committing their troops to useless offensives in their western front with France...
All throughout the war, the german high command failed again and again on strategy. They miscalculated that Belgium would fold, were caught unprepared when the bergians fought, and wasted days bringing heavy artillery to demolish the forts at Liège. They kept pulling troops from their planned invasion of France until their western wing was too weakened to be able to carry out the original plan. They also chickened out and failed to withdraw from Lorraine, as per the original plan, thus failing to draw the french main force (the french still being committed tho the offensive there) away from Paris into a position where they couldn't redeploy west in time. Finally, they started submarine warfare against the UK without first building a large submarine fleet (the UK actually had more submarines at the start of the war!) and without being fully committed (they backed out of it when faced with the possibility of the US entering the war), thus exposing that british vulnerability without exploiting it, and giving the UK tine to prepare countermeasures.
The Allies, otoh, didn't made nearly as much mistakes. Not pushing for Istanbul was one, but that the one big mistake I can think of.