Has the entire Northeast Division ever made the playoffs before ?

This is kind of a tricky question.

The only time in NHL that an entire division made the playoffs was the Adams division in 1981, so strictly speaking no, the Northeast Division never did it. However, the Adams could be viewed as a precursor to the modern Northeast division (indeed, in 1981, Boston, Buffalo, and Toronto were all in the Adams, and Montreal and Ottawa would both later join the division after Toronto moved to the Norris for a decade), so in spirit, one could say that yes, the equivalent of the Northeast managed to get all of their teams in the playoffs once.

Never have all teams in a single division made it to the playoffs in the same season, since the present division format was created.

In the season ExPat cited there were 21 teams in the NHL and 16 made the playoffs. Even the CFL doesn't have as good odds at playoff representation.

Actually in 2010-11 season the Dallas Stars finished two points and a tie breaker behind Chicago Blackhawks for eighth place in the West. If they had passed the Hawks all five teams in the Pacific Division would have qualified.

probably not considering thats where the leafs are