I don't think you need to do the quest to get in.
Have we actually completed the 500m? 
According to Nemma and the quest guide, yes. The 500m are needed to actually start the quest to open the gate and Nemma will tell something about some automatic security and so on, what she actually does. So yes, the 500m seem to be spent. But I dunno, maybe the GMs are planning something different for us...