PET will not give you tissue diagnosis, just tell you if it's malignant or not. Yes, a biopsy can be wrong. Needle biopsy is not enough, you need an excision. I'm puzzled why your bone marrow biopsy was normal, but your wbc was 1.5. Your symptoms are certainly consistent with lymphoma. Listen to your oncologist.
I do want to add, you can request your biopsy slides to be sent for a 2nd opinion. Mayo clinic, for example.
Well, I'm not a doctor but I have to say if I were you I would have another biopsy done, and not just FNA (fine needle aspiration). My personal feeling is if something isn't supposed to be there, TAKE IT OUT!! I'm not sure where you live, but please go to a major medical center in your area. I'm not sure why one MRI would show enlarged lymph nodes all over, but then you say CT, MRI, etc. "all ok" What are your liver enzymes? Have you seen a nephrologist? He may have answers to your hypertension, and positive ANA. Good luck!