Definitely not trading Severson this season...that would make no sense. View him as a rental.
With Hughes and Nemec likely on the roster next year, Severson and Graves are (most likely) both gone. Along with Bernier, Wood (I suspect), Johnsson, and maybe Tatar and Blackwood, the Devils will free up some cap space that should allow them to keep Bratt and Meier (if they go in that direction).
So Nemec/Hughes replace Severson and Graves at a big cap savings.
Johnsson and Bernier are dead cap space and can leave without needing replacements.
Wood and Tatar probably get replaced by a combo of Holtz, Boqvist, Foote, and Clarke. All the replacements are on inexpensive deals.
The problem probably starts when you need to pay Mercer, Nemec and Hughes. That's where some of the existing core probably needs to get shipped out.