First of all congratulations.
Murree & Nathiagali are quite easily accessible even in winters so yes you can very well go to Murree & Nathiagali. As far as other areas are concerned like Naran, Neelum valley, Swat etc i'd rather say you better skip those for now cux they wont be easily accessible. You can however, take a flight to skardu or gilgit and explore a bit of gilgit baltistan region but then again it will be extremely cold and accessibility will always be an issue.
As far as safety is concerned, Alhamdulila North is not a problem as such. You can freely move around, people are friendly and supportive in general. Murree & Nathiagali are quite populated even in winters so they're good to go as well.
P.s. Northern areas can best be visited between May-September with the exception of monsoon season cux the weather is pleasant and accessibility not an issue.