I will try and sum this up as simply as possible.
You have typical changes on your MRI scan which indicate degeneration, collapse and compression of the nerve roots coming out of the lower spine, causing you lower chronic backache which in essence is ...sciatica. The bony collapse of the lumbar and sacral vertebra, may cause other problems with your bowels and bladder...may....not necessarily. Now this epidural lipomatosis is not a really big deal and it is the growth of fatty tissue which adds to the compression of the nerves, already annoyingly undergoing compression...a sort of "double whammy".
Be reassured, there is no suggestion of any malignant process going on and your problems are purely degenerative...wear and tear.
Now the problem with you will be how to treat your back....long term medication OR spinal surgery to relieve the compression (laminectomy).
The cause of Epidural Lipomatosis is unknown.