What's the meaning of the title of Manstein's book, 'Verlorene Siege' - missed victories or victories won in vain?

If you only take the title, there are various possible interpretations as you explained in your question. But if you read the book, you will see that your interpretation 1. is the correct one. Manstein attributes the military collapse of the German army to Hitler's military incompetence (Hitler was officially the supreme commander). Actually Manstein wants to exculpate himself and the general staff. But he was an essential gear in the whole machinery.

Perhaps a better title would be "Verschenkte Siege".

Anyway, you should not read Manstein's book without seeing the context. Perhaps some defeats could have been avoided ("lost victories"), but the war could never have been won. And, more important, the war was not a "normal war" (sorry for that inappropriate expression), but its final aim was to exterminate the states of Eastern Europe and their human population. Hitler's aim was to gain "Lebensraum im Osten" (living space in the East). The German army was not a shelter of military honour as many people have claimed and still claim, but Hitler's instrument to achieve his criminal intents.

As a supplement to Manstein's book I recommend to watch the movie "Downfall" (in German "Der Untergang").

First of all: I did not read the book and I think no one should, since it is the memoir of a self-exposer/showoff and (likely) war criminal who found mistakes in warfare only with others. I won't add a link, but whoever wants to read it ... you will find it online.

The book has three "acts", "I. Der Feldzug in Polen", "II. Der Westfeldzug 1940" and "III. Im Kampf gegen die Sowjetunion". With this in mind "Verlorene Siege" may refer to all the victories (taking of Poland, France + huge parts of Russia) which where lost at the end of the war. As you said in your second guess.

---

But I think your first interpretation could also be correct. Wikipedia says

Manstein, who commanded the south German sector forces in 1943, complained that Operation Citadel, the offensive against the Soviet forces in Kursk, was delayed too long for the German force to break through. He also wrote that Hitler had halted the attack prematurely, a decision he called "tantamount to throwing away a victory". According to Manstein, Hitler (whom he praises and criticises, occasionally referring to him as "that dictator") did not allow the detailed planning of large-scale military operations. Manstein wrote that in 1943, a draw could have been achieved on the Eastern Front by bleeding the Red Army dry if the generals had been allowed to operate properly.

He talked about battles that could be won but were lost. Of course it wasn't his fault every time, but that of other generals. As @amadeusamadeus said you could call these "Entgangene Siege".

But to be honest: "Verlorene Siege" is by far the better title and oxymoron than "Entgangene Siege"

If we would take only the title itself, we would be able to rule out your first suggestion as intended main meaning, since "victories that could have been gained" would be entgangene Siege. However, it's not impossible that Manstein used this phrase that rings somewhat similar as a stylistic device, as others conclude from the context (I don't know the content of the book well enough to judge).

Your second suggestion could be true:

• Victories whose implications were later lost.

I would, however, suggest another meaning:

• Victories that were indeed gained, but turned out to be a defeat in fact (like a Phyrric victory)

Note that there are even more possible meanings: just as lost in English He's lost, verloren can also mean lorn or doomed. So Verlorene Siege can also be interpreted as:

• Victories that always were in a minority, 'lost' (lorn, lonesome, dispersed) in the overall battle
• Victories that had always been doomed, without prospect of leading to an overall victory

Manstein (or whoever chose the title) was probably aware of these manifold meanings and went actively for the contrast verlieren vs. siegen.