In degen. Disc disease, is it better to fuse l5-s1 if disc 1/2 collapsed or wait til disc completely goes & pain is unbearable & nerve damage occurs?

None. Usually, degenerative disc disease is not surgically corrected if no nerve impingement is present. Just treating back pain with surgery will cause more back pain due to scarring and presence of the presence at other levels. If there is nerve impingement and symptoms of it, then surgery will help free the nerve and improve the symptoms.
It's not that simple. Intervertebral disc degeneration is a normal aging process. It may cause pain because of "leakage" or most commonly because of abnormal nerve ending ingrowrh into the disc. Discography should answer question of exact nature of the problem. Fusion may not help in majority of cases.
DEGENERATIVE DISC. This is something you have to discuss with your orthopedist and your primary care physician, as you have to weigh all of your options based on all information available to you. I always say "surgery only take pain away if there is dislocation or fracture, no cure for pain" pain can be managed or controlled. Good luck.