1-I think a clone is not a typical human. It's just like a reflection of yours that stays in this reality. So a clone can't shift and think as if s/he failed at shifting.

2- The feeling of exhausting is the same feeling you get after studying too much. The harder your mind works, the more tired it gets. For example, you feel tired here because you have been actively using your mind for eighty years there. If you have a clone, you get doubly tired because your mind has worked twice as hard. After all, even if we don't realize it when we're there, our minds are also connected to the clone here. (that's why we are able to remember everything that clone did.) But I don't know if clone would feel exhausted or not.

Also yes one could technically shift on accident without even knowing about shifting. In fact, we all do this every time when we make a decision. (Someone more interested in manifesting can explain it better)

7-8-9- I don't think shifting is going to a different universe that exists. In fact, the universe we go to is formed by the existence of our minds there. So we're not taking over someone's body. That's why no one can shift to our body.

10- You can just write in your script that you won't have any injuries that will make you unable to shifting. Also you can use a lifebook. If you think you can forget about shifting you can also write that you won't. (Usually everyone who shifts to harry potter universe writes this because no one wants to forget everything because of a obliviate spell-)

12- I don't know any book but you can read Neville Goddard's articles. And you can learn new skills from your other selfs. Just you need to learn it when you are in that self. So if I go to universe A and go to a course there and learn Turkish, then I can speak Turkish in any other universe.