In theory, yes, if you do Season Unending and get the temporary truce, once you finish out the MQ you should be able to pick up the CW questline where you left off and play it out from there. But as noted previously there are sometimes issues with doing that depending on where you were in the CW when you decided to initiate Season Unending. My last PC asked for and got the truce before joining either side in the war because I didn't want to risk fouling up either questline by having them cross paths too many times.
All you can do is try it and see what happens. Go back to Tullius and see if you get the dialogue options for returning the Crown. If not, well you can alway hie thee to Windhelm and see if Ulfric still wants it.
(J/K unless you really want to switch sides just to unload the damn thing.)