It is fine and good if you are doing it with purpose (e.g. tutoring someone else in your mind to understand something better, or trying to work out some problem that has a solution), but bad if you are trying to solve something unsolvable, dwelling on the past or worrying about the future, or just wasting your own CPU cycles.
This is something you can practice as part of mindfulness practice - getting spontaneous awareness that you are engaging in an unproductive internal dialogue and letting it go. Not fight it or stop it, but just let the thoughts come and go without engaging with them.
But I disagree slightly on #3. My frame of reference is thru-hiking and/or intense solo missions where I'm alone in the backcountry hiking for 10-14 hours a day for days on end. In that setting, I find it impossible to stay mindful continuously. Also, without news/email/social media/etc. as input, I just find that my mind gravitates towards internal dialogues about the same big picture life issues. In that state, I'm actually listening to my body and mind more, not less. Personally, it can be frustrating to worry about whether I'm "in the moment" enough (the quintessential frustration with mindfulness, I know).
That said, I do try to mix it up between fully immersing myself in the wonderment of the wilderness, letting my mind chew on ideas, listening to podcasts/music, and sometimes attempting actual meditation sessions.
One definitely should not be frustrated or worrying about being in the moment! I agree that it is all normal in the sense that one shouldn't be worrying about any of these habits. It's just that for me it was a revelation when after many many years I realized that I do not have to have/follow internal dialogues and that they may actually causing me tension and anxiety.
The introductory course on the Headspace app I also found good but I think it is better to not stay on it beyond that.
If your thoughts keep you awake sometimes I think you would definitely benefit greatly from this.
- replaying some sort of argument/confrontation or other high-stakes conversation from earlier in the day (simulating ways the conversation could have gone better--the best zingers are the ones you come up with in bed at night).
- preparing for an important presentation or meeting the next day (informal rehearsal).
- out hiking alone (especially longer solo backpacking trips). Without fresh content bombarding my brain, I tend to dwell on the same ideas/conversations.
- reading a particularly engrossing book, and get into a character from their point of view.
- studying something new, then I imagine myself tutoring someone else on the subject (my go-to learning technique).
Now if you find yourself having conversations that you were convinced are real at the time, but it later dawns on you that it never actually happened, then that might be something to talk (for real) to someone about.