I think the problem with people who are new is what's a 'little sore'? A person who has never lifted will have DOMs that is going to feel a lot more than a little sore after their first workout. It's completely normal, and not dangerous. In fact, a 30 minute stationary bike ride will probably clear it up.

It's actually really hard for someone to overtrain. What normally happens is people use it as an excuse to stop training.

What I tell people is that long term training is a grind - and that's a good thing. Accept that fact early on and find your own enjoyment in it however you have to. The discipline that comes to other areas in your life from accepting the training grind is a benefit many people don't even notice.

I disagree. The enemy isn't not training hard enough. The enemy is burnout and quitting. Train to the point where getting back into the gym requires as little discipline as possible.

There is, especially in weight rooms, a danger of doing stuff that's not really exercise. But if you're swimming, running, climbing, squats, pushups, pullups, tennis and such... it's not really a danger. They're all going to put a beginner on a meaningful progression path. Don't worry about being insufficiently intensive unless your progression has halted.

The grind isn't about training hard enough, it's about consistently training. Doesn't matter which mix of exercises someone decides to do (it does, but assuming the opposite is zero), consistency will eventually become a grind.

You're right the main enemy is quitting. When I tell people I workout nearly every day, people have this illusion that it's easy. It's not. I make time for it in my day, I make it a priority. That's what I mean by the grind. Everyone is different, but I find embracing the uncomfortableness easier than looking for ways around it.