In my opinion, 7 weeks is too early for an elliptical machine. Until the fusion level stiffens (at least 3-4 months), no up and down loading should be considered. A stationary bicycle and simple walking with hiking poles and pool walking should be the order of the day. Even though you don’t have a disc, you have a spacer (“stone”) wedged between two bony vertebra held together by thin ligaments and screws in the back of the spine (hopefully) which cannot withstand up and down pounding. I would back off.
Donald Corenman, MD, DC is a highly-regarded spine surgeon, considered an expert in the area of neck and back pain. Trained as both a Medical Doctor and Doctor of Chiropractic, Dr. Corenman earned academic appointments as Clinical Assistant Professor and Assistant Professor of Orthopaedic Surgery at the University of Colorado Health Sciences Center, and his research on spine surgery and rehabilitation has resulted in the publication of multiple peer-reviewed articles and two books.