Will completing the Dark Brotherhood quest chain hinder any significant quests/events?

Solution 1:

No, none of the Dark Brotherhood's quests intersect in any meaningful way with any other quest chain. Some targets may have minor 'radiant' quests available, but none are significant in any way.

The worst thing that can happen is you can end up with a very large bounty on your head, which could make completing other quests... difficult at best. However, this is both a solvable, and preventable problem.

The only collision other than the one you've noted, is that

Vittoria Vici, the target of the main-line quest Bound Until Death is also involved in the incredibly minor quest The Spiced Wine.

Solution 2:

For the side contracts offered by Nazir, none of the targets offer anything unique except for Narfi (whom you've mentioned). Some of them, like Betild or Anoriath, will offer or be the target of another radiant quest (mine ore, companions quest, etc), but Narfi is the only one with a unique one.

Some of these targets are vendors, but none of them are particularly useful to adventurers (Anoriath sells 10 random pieces of meat, for instance), nor do they have particularly large purses.

Finally, the Dark Brotherhood Radiant quests (after completing the quest line) use generic NPCs, such as "Noblemen", so once you've completed the initial quests, you will not be tasked to kill random named NPCs.

My advice? Do Narfi's quest, then put the poor fellow out of his misery, as gently as possible.

Solution 3:

I would say no, other than if you join the Dark Brotherhood, you very well can't Destroy the Dark Brotherhood.

Solution 4:

Yes, it will. Blood on the Ice is a fairly major side quest taking place in Windhelm. I consider the quest to be pretty important part of the lore of the town.

Although most of the NPCs needed for the quest to initiate are marked as essential, another quest, namely the Dark Brotherhood quest Mourning Never Comes, will encourage you to kill Nilsine Shatter-Shield. Even though she is not related to this quest, her mother, Tova Shatter-Shield, will commit suicide if Nilsine is killed, and will therefore render this quest impossible to initiate.