"It was a mark of how bad the last week had been that the other two agreed with him." meaning in this context?

Question

There was a tap on the dark window.

“It's Hedwig!” said Harry, hurrying to let her in. “She'll have Charlie's answer!”

The three of them put their heads together to read the note.

Dear Ron,

How are you? Thanks for the letter — I'd be glad to take the Norwegian Ridgeback, but it won't be easy getting him here. I think the best thing will be to send him over with some friends of mine who are coming to visit me next week. Trouble is, they mustn't be seen carrying an illegal dragon.

Could you get the Ridgeback up the tallest tower at midnight on Saturday? They can meet you there and take him away while it's still dark.

Send me an answer as soon as possible.

Love, Charlie

They looked at each other.

“We've got the invisibility cloak,” said Harry. “It shouldn't be too difficult - I think the cloaks big enough to cover two of us and Norbert.”

It was a mark of how bad the last week had been that the other two agreed with him. Anything to get rid of Norbert — and Malfoy.

I don't quite understand the last two sentences in bold above, especially the first one. The sentence seems to be saying: it was a mark that the other two agreed with him. But I don't get what it is trying to convey and what "a mark" really is? What's the correct way to understand them?

--Excerpted from Harry Potter.

Answer 1

The key is actually the sentence before:

“It shouldn't be too difficult - I think the cloaks big enough to cover two of us and Norbert.”

Harry thinks that a single cloak can cover 2 children and a dragon. This is, on its face, absurd. This plan being slightly insane and difficult is the key point, as we'll see later.

...the other two agreed with him...

They agree with Harry that the cloak will work (even though, as we noted before, most people would say this is crazy and unlikely to work)

It was a mark of how bad the last week had been...

The previous week had been so bad, that Harry's plan actually seemed like a decent idea.

Anything to get rid of Norbert — and Malfoy.

Ron and Hermione were absolutely desperate to get rid of Norbert and Draco.

So, we can essentially interpret this passage as:

Harry came up with a plan to cover two people and a small dragon with the invisibility cloak. This was not a very good plan. However, because last week was bad/insane/crazy and they are desperate to be rid of Norbert (and to get Draco off their back), Ron and Hermione agree to Harry's reckless plan.

Answer 2

It seems to be saying that:

The fact that the other two agreed with him was indicative of how bad the last week had been.

"It" in the beginning refers to the act of agreeing.

"A mark" is another word for "an indication"