#1 this is the intended default behavior of the importer. By default it does the following:
- Do not create MeshPart assets for each individual mesh (that is what shows up in the mesh tab)
- Create 1 Model asset for the entire scene (the entire contents of the FBX/glTF is contained within this one model)
The goal with this default behavior is specifically to reduce unwanted clutter. E.g. if a 3D file has 100 meshes it only creates 1 model, if it has 1000 meshes it still only creates 1 model.
If you would prefer to see each individual mesh as its own asset, you can uncheck the option Import Only as Model. Doing so will (in addition to creating the model asset) create a MeshPart asset for each individual mesh. In the case of the 100 mesh file, you will get 101 assets: 100 MeshParts, and 1 Model.
You can also disable creation of the model asset by unchecking Upload to Roblox. Disabling this option will simply insert the model in your workspace after import completes. This comes with the downside that if you delete the model from the workspace you will not be able to re-insert it from AssetManager or Toolbox (as no asset was created).
So if you would like 0 asset clutter as a result of import, make sure Import Only as Model is checked, and Upload to Roblox is unchecked.
And with this new update, you can save these setting as a preset and make it your new default 
#2 work is currently being done in this area so things may change in the future. But for the moment, Roblox assets that are not packages are considered immutable, allowing updated or deletion would break that assumption and break our systems.
As for editing meshes, the new EditableMesh API allows for building tools that do exactly that, check it out: Introducing in-experience Mesh & Image APIs [Studio Beta]