Sorry, but recommending St. John's Wort is NOT a good idea! St. John's Wort has been proven (in peer-reviewed scientific studies) to prevent birth control from WORKING! It stops birth control from preventing births. That chemical in Mirena that you're referring to that you say St. John's Wort will stop from working--it's kinda the whole point of using Mirena in the first place.
Other than that, some of the advice you mention may be worth trying. I think you're right and that Jen's body is just going through a readjustment period. She should do some research on her own and see if any of your other recommendations may work for her.