I didn't say it's necessary. It is beneficial. Since you haven't had the opportunity to taste this particular "chocolate," there's no way to really convince you. What you say about Guru Yoga is, theoretically, true. However, being live in the Guru's presence makes it all so much easier. At least that's my experience.
In terms of your second point, I was actually referencing something that Tulku Sang-ngak Rinpoche said last January during a drubchen at Rigdzin Ling/Chagdud Gompa. He Himself was saying how He would often feel confident about a practice until He was on the cushion alone in retreat and then some detail He had never thought about before suddenly needed clarification. This struck a chord with me since I have had this experience several times during retreats. However, I had in mind fairly complex Mahayoga practices with lots of details. If the practice is simple, then sure, maybe no questions on how to do it.
Good luck and best wishes.


