The idea of Nanowrimo is to get writers to try and write a 50k novel. A first draft if you will. And most first drafts will not be perfect, they'll be the furthest thing from a great webnovel. It'll probably have plot holes, grammar mistakes etc. So, to answer your third question no.
It can be a good idea for your webnovel if you want to complete or at least get most of it written.
I am participating in Nanowrimo but that's only because I want to try the 50k word count. I'm using the challenge to fill in projects that I have, trying to complete some. As for other people, they plan a project and write it during November then use December and next year to edit and improve it before publishing it.
Honestly, what you do in Nanowrimo it is up to you, but the whole idea is write 50k words in a month.
