Harmlessoldlady,
Making the lots with quests on them, in China, "No Visitors Allowed" does not interfere with quests. Whenever I start a new game, the first thing I do, from the home "edit town" screen is change the problem lots in China. It stops NPC using these lots, but not your sim. The only problem lot where you cannot do this is the one below the Market. This can be a problem as the shop staff sometimes go into it and can't find their way out. Making sure that you find the hidden back staircase helps though.