Glad you are enjoying mansions and this is all down to personal perference.
1. Get DLC if money is concern since expansions can be expensive ( I'm from Canada so big Box expansion such as Arkham and Horrific are about $70). Generally Streets of Arkham is regarded as the best expansions since it has a mixture of great invesigators, mechanics will can help the player and fanastic scenarios.
If you enjoy Mansions for the thematic setting, I personally believe that the Beyond the Threshold expansions offers the best scenarios, followed by Horrific Journey, Streets of Arkham, then Santcum of Twilight.
On the other hand if you are a "power gamer" and you are concerned about making the most optimal play for success ( choosing the best investigators, items mechanics or cards to help in your battle with Cthulhu), then the expansions order is Streets of Arkham, Santcum of Twilight, Horrific Journey , then Beyond the Threshold.
2. You can House rule not using the traitor mechanic if you wish and also it does not apply to fewer than 3 investigators. Don't want to go into too much spoiler detail, but the specific scenario can be quite challenge even without a traitor.
3. Yes the tiles and monsters of expansions due appear in all of the other base/DLC scenarios . The first scenarios "Cycle of Eternity" has may map variants depending on which expansions added.