Its complex but once you get the hang of it, it starts to make sense (for the most parts). On average i estimate that you need about 3 month to get familiar with the editor. For most questions are already answers available, and if not you get normally a quick response via the forum or at Unreal Answers.
Hmm. Basically for collision you need a collision capsule. That is for characters, for static meshes or foliage, set the collisions to “Block All”.
You can create collisions inside each mesh with the collision tool (simple or convex). If this is not possible for a particular mesh, export the mesh, then reimport it, or maybe try the recalculate option inside the mesh editor.