Time Words – Meanings of ‘Till’ and ‘Until’ Explained

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.

I'll break the passage up into two parts to address your two questions.

I can’t be bothered to spend much time on her until she passes the five-year mark, …

The narrator is using mark here to mean

Noun. a figure registering a point or level reached or achieved

— https://www.merriam-webster.com/dictionary/mark (sense 3h)

So the “five-year mark” is a goal of having been her father's wife for five full years. Note, Séverine almost certainly doesn't know about this goal; there is no general notion of a five-year mark as anything special. Which brings us to

…, Dad’s record to date

This is an appositive, so the phrase is describing the previous phrase (“five-year mark”). Here the narrator is using record to mean

Noun. an attested top performance

— https://www.merriam-webster.com/dictionary/record (sense 3b)

and to date to mean

up to the present moment

— https://www.merriam-webster.com/dictionary/to%20date

The narrator is explaining why she chose five years in particular: because that is the length of her father's current longest marriage. But she is also commenting on her father's relatively short marriages. One would usually use the word “record” to refer to something extraordinary or impressive — e.g., the Guinness Book of World Records — but of course five years of marriage isn't particularly extraordinary.

Putting the the parts back together, the narrator is saying she can't be bothered with Séverine until she and her father have been married at least as long as his previous longest marriage. But I get the sense that this is really just sarcasm; the narrator is giving a hurtful and dismissive reason to ignore someone she doesn't really like, while also remarking on her father's serial monogamy.