The answers to these questions depend a lot on what you are aiming for. You are not going to want to use the base android sdk for game development. You are probably going to want to look at other tools/frameworks like Unity or LibGDX. They will get rid of a lot of boilerplate code and make prototyping/development faster.
1) Depends on what toolset you choose. You might choose an engine that abstracts out a lot of the underlying stuff like Unity. If that's the case you probably won't have to look at the native code. It would still be useful to learn for making the final builds or debugging if issues do come up.
2) Depends on what your timeline is. If you want to release a game in like 4-5 months you'd definitely want to go with a pre-made engine. If you absolutely must write your own engine be prepared to spend A LOT more time.
3) Depends on what toolset you choose. The best tools will have a WYSIWYG editor, the format the tool exports is irrelevant. If you write your own engine.....well then you are going to need to write your own editor.....the last thing you want is for your artist wasting time editing XML files.
I think the best thing for you to do is evaluate what you want to accomplish. Then research the best tools to use to get you there. Experiment with a bunch of tools until you find one that suits your needs. Give each tool a good week of experimenting like trying to create a simple asteroid clone in each.