If pvp is immenent in front of NPC's, but no DM is available/online or responding, is it then not allowed?

The players should know OOCly that PvP infront of NPCs is metagaming them not to react as they might. Hence the rule of not PvPing infront of NPCs with no DM on to take control. Do it somewhere else, there are plenty of spots both in the city and an astounding amount in the underdark where this can occur.

Thanks for the clarification.

There is some leeway for law enforcement factions when they're in their territory.

If you engage in PvP in the streets, we always ask for a DM tell.

However you may engage in PvP with no DM around if your character is in a law enforcement group (Watch, Spellguard) and you need to do so as part of your duties in an area where the NPCs are very friendly (The Rock Bottom, the Watch House, the streets around the Town Hall). Of course, if your law enforcement PC actually is in disguise and trying to assassinate someone, then you need a DM there.

If you're unsure about whether an area is "safe territory" contact a DM.