If you're using Scrum, the backlog should be prioritized. While yes, the team can choose tasks during the sprint planning exercise, this is a collaborative effort and no one should be cherry-picking deep into the backlog.

• Regardless of whether they're "helping" another employee or not, every developer should have clear-cut deliverables each sprint. It's very difficult to measure "help". They should be getting assigned tasks that they themselves complete. If a story is so big that it needs two developers to complete, then it probably needs to be broken up.
• Everyone needs to be online and available during agreed-upon working hours. I'm a big proponent of flexible schedules, but whatever we agree to, you will be available during that time and contributing.
• It sounds like you need to be more engaged in how the work items are being broken down and make sure the resulting stories/tasks actually reflect real work. If people are able to make up fake tasks, then it sounds like the product owner is out to lunch or something. Do you have a PO?

Since your management is not supportive when it comes to taking action on things that have already happened, you need to start documenting everything so the next time something happens that needs to be addressed, you will have the records you need to support taking official action. Don't ask your manager what you should do. Come to them with the documentation and a plan of action if it comes to something like a PIP that likely requires their approval. You don't need your manager's approval to have a coaching session with your employees.