A surviving Kalmar union is more than possible, though not always likely. I think that it would need a Danish or Swedish king who is willing to focus his reign solely on integrating whechever of the two he isn't from. Norway is almost a nonfactor, and would thus be easily controled by whichever of Denmark or Sweden is stronger, but you would need serious effort by one or two strong rulers for whichever state the ruler does not hail from to accept him.
As for long term prospects, beating Britan one to one in anything is unlikely, but the Kalmar union could be a useful ally to France or Spain when they come a knokin' at England's door, and may even be able to land a successful invasion given how they are on the opposite side of England compared to England's traditional enemies. United Scandinavia could also benefit tremendously as a British ally, supplying naval supplies in exchange for the right to colonize certain territories, and perhaps threatening to withhold those naval supplies to extort more money during war times. The only big problem is Germany, but with a different reformation, a butterflied Prussia, or any number of other things Germany could become friendly or neutral towards the union, so Germany is a danger, but not an inevitable death sentence.