Difficulties with behaviour of other PhD students in group

In my experience it is not uncommon for people in your situation to not "gel" perfectly with the rest of their cohort. You are used to an industrial way of working, which is, or at least should be, quite professional and focused on a common goal (e.g., shipping some product). You are also used to fairly clear management structures, as indicated by your usage of "reprimand" and "misconduct procedure". Academia typically does not work like that. As user2768 aptly observes, a PhD study is an individual project - you are not all together shipping some common product, but you are all working on your individual research (or, in the worst case, you are directly competing for papers, grants, and future positions). That does not mean that you can't or shouldn't be friends with the people in your cohort, but it certainly gives the interactions a different dynamic.

I have found the following to be a good mental model for working in a research lab: envision yourself as a one-person (or two-person, you and your supervisor) early-stage start-up company working in an incubator or shared office space with other start-ups in a similar position. Sure, you want to have a nice relationship with these other "start-ups", and often it will help to collaborate on this and that, but ultimately you don't need to work with any of them. If you feel it is more productive to do things on your own, do so - and, conversely, don't feel that they are obligated to help you with anything specific you need.

As for how to deal with the specific behaviors you mention: essentially, my tip for all of them is to tell the respective students (more or less friendly, depending on your personality and whether you would like to build up or retain a friendly personal relationship) to bugger off and annoy somebody else. You don't need to listen to them giving you unwanted feedback, you don't need to convince them of your opinions, and they don't need to explain how they arrived at a specific solution. If a specific student annoys you or is categorically unhelpful, stop interacting with them.

With all that said, I feel there is also a good chance that parts of the conflict are also related to your own mindset. I think it would help you to stop seeing yourself as mature and them as immature and in need of correction. While I am (based on your descriptions) convinced that some of your colleagues are indeed not the nicest of people, you won't be able to change them. It is more productive for you to take a hard look at your own behaviors, and whether you are potentially not as adapted to being back at university as you would like. Maybe letting go of a notion of superiority and maturity, and making some corrections in how you deal with your colleaguesm will also help normalize your relationships.

Edit: Some specific advise on dealing with defeatist people. Yes, those are a problem. Avoid very negative students at all costs. Nothing kills your motivation to do challenging research quicker than a friend who talks about nothing else than how pointless their research, your research, and a PhD in general is. Make very clear that you are not interested in that kind of conversation, and if they persist stop interacting with them.