As the answer in the previous question said, mangakas are ahead of 3-4 chapters though this figure may vary in some cases. The credible source for this is Eiichiro Oda, creator of One Piece manga, answers his fans questions at the SBS Corner, here's what he had said about this in particular:
D: When an artist is drawing for example, Chapter 10, what chapter is
being printed in the newest Weekly Jump?
O: I see. I wondered about
this myself, years ago. So I understand your curiosity. At this EXACT
moment, Issue 46 of Jump is hitting the stands. It contains One Piece
Chapter 60, "Solution". But I have finished the drafts up through
Chapter 63, "I Ain't Gonna Die". So a chapter comes out roughly 3
weeks after I finish it. But that's only my CURRENT schedule. Not all
people drawing weekly serials have the same system. I could change if
need be. There are various cases.Source
And for your other question of how far ahead the mangaka develops the plot, Hiro Mashima, the mangaka of Fairy Tail says:
Q: How far in advance do you create your stories (before they're
published in Shonen Magazine)?
Hiro Mashima: Generally speaking, I tend to think of the next episode
as I'm creating the current one. Sometimes I get writers block.
Sometimes inspiration just comes when you're sitting down at the
toilet. I like to think of that as just an inspiration from heaven.