I concur with la piquante's answer to your first question.
His answer to your second is semantically correct, but grammatically inaccurate: the it here is a 'dummy' employed in a cleft construction.
However, the sentence is very complicated. Note, first, that there are two distinct propositions here:
A. If Voldemort is delayed again and again ... (then) he may never return to power.
B. To delay Voldemort will merely take someone prepared to fight what appears to be a losing battle next time.
Let's simplify those radically, so the syntax becomes clearer:
1. If [ DELAY1...n ] then [ DEFEAT ]
2. [ DELAYnext time ] will merely take [ HERO ]
Dumbledore's sentence is built on Proposition B, recast as an it-cleft:
It will merely take [ HERO ] to [ DELAYnext time ]
At this point a common English construction we might call ONLY... AND comes into play. This construction is syntactically very similar to the IF ... THEN construction, and bears a similar interpretation: ONLY do this, AND that will happen. The ONLY element which introduces the condition clause may be any adverb or adverbial with the approximate sense of only—merely, just, but—and the consequence clause is introduced by and. These constructions are often used to motivate or warn:
Only try, and I'm sure you'll succeed!
Just mess with Big John and you'll see what happens.
But screw your courage to the sticking-point and we'll not fail. -Macbeth
This construction marries very readily with verbs like require/need/take:
It only took one whistleblower, and the whole scheme collapsed.
Dumbledore does just this. He moves the next time into the condition clause, where merely acts as the ONLY element, and combines the AND clause with the Proposition A conditional:()
It will merely take [ HEROnext time ] and if [ DELAY1 and DELAY2 ...] (then) [ DEFEAT ].