As was pointed out on that other question you linked, this is a surprisingly tricky sentence!
Let's slowly build it up.
It will take someone else.
What is "it"? Stopping Voldemort again. Implied but never directly said. As for "take", we could say "need" instead, just to be slightly clearer.
[Stopping Voldemort again] will [need] someone else.
This isn't "someone else" as in "we need a different person", but as in "we need other people prepared to do what you did, Harry". When will they be needed? "Next time" (that is, the next time that Voldemort tries to return).
[Stopping Voldemort again] will [need] someone else who is prepared to [do the same as Harry] next time.
Now, Dumbledore is saying that this someone else doesn't have to do much. Hence, it will "merely" (or "only") take someone else. This may sound like it's demeaning Harry's efforts, but it's meant to be reassuring Harry: standing up to Voldemort is not very difficult or unlikely after all, and it doesn't take someone extra-special to do it.
[Stopping Voldemort again] will merely [need] someone else who is prepared to [do the same as Harry] next time.
What, exactly, did Harry do that Dumbledore says they need other people to do (or be prepared to do)? "Fight a losing battle"--or what seems like a losing battle. (Dumbledore omits the word "like", but I'll leave it in for this one example.)
[Stopping Voldemort again] will merely [need] someone else who is prepared to fight a losing battle next time.
[Stopping Voldemort again] will merely [need] someone else who is prepared to fight what seems [like] a losing battle next time.
And now we just replace the bracketed bits with the different wording Dumbledore uses, and we have the sentence (okay, part of a sentence) that you bolded.
It will merely take someone else who is prepared to fight what seems a losing battle next time.
You are on the right track. Will is the subject; it is a noun in this sentence.
The second half of the sentence is written in a nonstandard dialect form. In standard English, it would be "...that nobody can meddle with it."
Best Answer
Very simple words like "play" or "run" or "go" have so many meanings that it's often difficult to tell which is the correct one for a given use. In this case, play means to utilize or exploit. From the Merriam-Webster definition:
So in this sentence, the author is stating that extortion hacks work well because they are able to make use of a company's fear of lawsuits, etc.
I'm not sure that I would have chosen play to here though. In a negative sense, it's almost always play on. When someone plays to someone else, they are generally behaving in a way that is calculated to gain favor from the target of the behavior. You might play to an audience of chemists with science jokes. That would be Merriam-Webster verb definition, 3.d, if you're curious. It's possible that the author was trying to use this sense of the verb, but it's not the typical context you would find it in.