1-Yes as for precaution a person should close the Tab or shower after completing one part of the body during Ghusl or if he/she is under shower come out and then wash another part of his/her body.
2-During Ghusl water should reach to all body parts and for that purpose sometimes we use our hands to make sure all parts are washed well otherwise if water can reach without doing that it would be sufficient even if we don’t rub our hands.
3- Yes you can do that.