DAV: Necropolis Halls Chests - Can I Revisit After Doing The Emmrich Quest?

I'm in the middle of the quest to recruit Emmrich, and have noticed several quests that "seem" unobtainable at the moment, whilst I'm doing the quest. For example, there's some doors that are locked behind those red crystal things, and I can't access the unshielded red crystal to destroy it and open the doors.

There's also some chests that require wisps, but I can't find the wisps anywhere.

I've been trying to figure out ways of accessing the doors/chests whilst I'm doing the Emmrich quest, a I don't want to complete the quest and then be locked out of the area. Will I be able to go back to the area after the Emmrich quest is completed, and do I in fact NEED to complete the Emmrich quest in order for the wisps and red crystals to be available?

Thanks.