My goodness, but these are some fascinating questions!
Regarding "lists", believe it or not, in the US it's a "generational" thing. I'm old enough to have been taught that one
does put the comma in, so: "He left town with lock, stock, and barrel in hand".
Regarding your second example, the use of a comma really depends on how much of a "pause" you want to put in the sentence. If you say "I picked up the spoon and stared at it for a while" (i.e. no comma), you give it a clipped, direct sort of impact--this is the kind of thing that Hemingway aimed for. On the other hand, if your character is in a thoughtful, or "dreamy" state, you might want to increase the length of the pause, something like "I picked up the spoon...and stared at it for a while". In short, it all depends on how you want to "pace" you narrative, not on some hard and fast rule.
Of course, your last question is the most interesting one. To what degree an author should "participate" in his/her narrative is something that has been debated ever since people started telling stories around campfires. To this day, opinion varies wildly. On one hand, you have authors such as Mario Puzo claiming that one should
never write in the first person! But what does this imply for something like, say, Dickens' "David Copperfield", a "personal history" which is, as you know, narrated in the first person? Conversely, there are authors who favor
extreme detachment from their stories, pointing out that this distance gives them the freedom to comment not only on their characters' behavior, but also on their thoughts and states of mind. Take, for example, "The French Lieutenant's Woman", wherein the author actually comments
cynically on the behavior of his own characters!! I suppose it all depends on how much you want to distance yourself and/or your readers from the action and the characters of the story: using first person, you are "in medias res", right in the thick of the action, but you can only comment on your (or your character's) own thoughts; using the third person, you can comment or describe
anything (e.g. in "Notre Dame de Paris" [="The Hunchback of Notre Dame"], Victor Hugo takes
an entire chapter to describe medieval Paris, which he actually marks "Pas pour le lecteur pressé" [="not for the hurried reader"]!).
So, it all comes down to what makes you the most comfortable.