1)Can a Sefer Torah be taken out of the aron kodesh so that a bar mitzva boy can practice the leining (in order simulate the environment that he will be leining in)?
2)Can a bar mitzva boy, when learning how to daven Musaf, use the Shem Hashem mamash (so that he doesn’t get confused when he actually davens for the amud)?
1] It is permitted to take out a Sefer Torah for a Bar Mitzvah boy to practice on, but if possible it is preferable to use a Tikkun Kor’im instead. [Note: Taking out the Sefer Torah for practising is only permitted in the Shul itself, or if the Sefer Torah is being kept in a private house. However, one may not remove a Sefer Torah from a shul, or from its fixed place, for private use – see Shulchan Aruch, Orach Chaim, Siman135]
[מקורות: עי’ ילקוט יוסף או”ח ח”ו עמ’ קלח ד”ה ויש, שהביא דו”ד בין הפוסקים בענין הוצאת ספר תורה שלא לשם קריאה בציבור ושדעת רוה”פ להקל בזה במקום הצורך].
2] From the question in would appear that the boy is not yet fluent in his reading of Teffilah. In that case, he may practice the brochos saying the name of Hashem. Similarly he may practice the brochos of Krias Hatorah and Haftorah in order to learn how to say it properly. This is permitted as part of the Mitzvah of Chinnuch.