It’s indeed the ett-form. You can read more here

If you speak German you can associate it to 'Notwendig'

nöd = need, -vändigt is I expect a common adjective word particle

if something gives you ' no advantage' then it is not necessary

Yes, if you're talking about a specific common-gender noun.

It's from the German notwendig, formed from Not meaning distress or crisis, and wendig meaning versatily or flexible.

We have nöd as a separate noun with the same meaning as Not, but vändig only really exists as a part of a few other words in modern Swedish.

No, it's only the pronouns mig, dig, sig that have this strange pronunciation.

That would have been Är det inte nödvändigt?

Often, yes, but this is a question - "is it?", not "it is".