Hi Linn,
I can't really answer your questions other to say that I think we all felt a little vulnerable finishing Herceptin which is normal, I found the longer out we are the more confident we become and it helped me to know my Onc would order any scans or tests needed if I had a n ongoing symptom.
What worked for me was to simply assume the chemo etc had done its job and now I had to get on and do mine which was living, and finding this obscure new normal I read about!
Try not to dwell too much on all this, you've done all you can and IF the worst happens you'll deal it with it then as you did now, but worrying too much is allowing the cancer to take over and win I think, try to go forward with confidence and stick to reputable sites is researching!