The line-comments depend on the commit that were made on.

If the person that made the pull request rebased that commit then you're not looking at what you were previously looking. It's a different commit, thus there are no comments.

Look on the 'Your Actions' tab and find a line in the actions' history were you commented on that commit, click that commit and you'll see the comments are still there.

You must create templates on the repository's default branch. Templates created in other branches are not available for collaborators to use. You can store your pull request template in the repository's visible root directory, the docs folder, or the hidden .github directory. Pull request template filenames are not case sensitive, and can have an extension such as .md or .txt.

You need to be sure that your base branch has:

• the 'Protect this branch' flag activated
• the 'Require pull request reviews before merging' flag activated
• the 'Require review from Code Owners' flag activated

Then in the CODEOWNERS file, located either in the root of the repo or in the .github subfolder, you need to have either a user described with '@user', a user described with his email like 'user@domain.com', or a team described with '@org/teamname'.

You will need also to be sure that:

• your contributors (users and teams) have for each of them the 'write' access to the repo
• the team is not a 'secret' team, only the 'visible' teams would be taken into account

Yes, it is now possible.

Add a file named pull_request_template.md to the root of your project:

- [ ] Have you done x?
- [ ] Have you done y?
- [ ] Have you done z?

You can also create a template for issues using the same convention. Just name the file issue_template.md.

Source: https://github.com/blog/2111-issue-and-pull-request-templates

If you want this workflow to run the job for everyone except for specific users, an option could be to add an if condition to your job to run only if the github.actor from the Github context isn't among a list of users you set.

Example with your workflow:

on: [pull_request]

jobs:
    build:
        name: Comment a pull_request
        runs-on: ubuntu-latest
        if: ${{ github.actor != 'Slava' }}
        steps:
            - name: Checkout
              uses: actions/checkout@v2

            - name: Comment a pull_request
              uses: mb2dev/github-action-comment-pull-request@1.0.0
              with:
                message: "Hello, Thank you for this PR!"
                GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

This also work for many users if you add && or || inside the condition.

For example, I use this workflow with some of my repositories to add PR comments:

name: PR Comment

on:
  pull_request_target:
    types: 
      - opened

jobs:
  PR-Comment:
    if: github.actor != 'user1' && github.actor != 'user2' && github.actor != 'user3' && github.actor != 'user4' && github.actor != 'user5'  
    runs-on: ubuntu-latest
    steps:
    - name: PR Comment
      uses: actions/github-script@v2
      with:
        github-token: ${{secrets.GITHUB_TOKEN}}
        script: |
          github.issues.createComment({
            issue_number: ${{ github.event.number }},
            owner: context.repo.owner,
            repo: context.repo.repo,
            body: ':warning: Have you followed the [contributions guidance](<contribution_guidance_url>)? Content PRs should generally be made against the the [source repo](https://github.com/<owner>/<repo>).'
          })

A full workflow example can be found in this repo

I don't think it's possible to use or inform the Github Team directly with a condition yet.