How to reuse complete paper for my thesis?
For the first question: When the chapter is really just your paper (reformatted), I would go with what you already suggested; begin the chapter with a small text saying "The content of this chapter has been published in ...".

If it is an extended version, then "The content of this chapter is an extended version of ...".

Regarding the second question: first read the copyright transfer that you signed. Often, it already gives permission for reusing the text and figures in your own thesis. If it does not explicitly state this, or is ambiguous, then it is best to contact the journal.