You are now 2.5 years out from a microdiscectomy of L5-S1 right with a “new” recurrent disc herniation “right disc herniation pressing on the sac, pushing the root of S1 in the canal space on the right as well as on the root of L5 out the right foramen”. This disc herniation is also compressing the right L5 root in the foramen. You still note symptoms of S1 (and maybe L5) nerve compression or dysfunction.
You also note discogenic pain “I still have problem with sitting on straight chair mechanical pressure sets in shortly after followed by discogenic pain after 1-2 hours” but you do not differentiate this pain from possible radiculopathic pain (nerve pain that appears to be unilateral lower back pain). Is your back pain central (both sided) or only unilateral?
You then have questions.
1. “will I’ll be able to sit more time freely without any mechanical pressure causing discogenic pain? i.e does “no motion no pain” principle applies mainly to vertical disc motion (instead of shock absorption in a normal healthy disc)?”. If you are having discogenic lower back pain and that pain is limited to the L5-S1 level, a fusion (TLIF, ALIF or OLIF) should reduce substantially your lower back and leg pain (as long as a decompression is performed with your fusion). However, if your unilateral lower back pain is nerve and not disc generated, a simple decompression without fusion can be considered.
2) “From a nerve recovery perspective is there any medical logic of fully decompressing the nerves 3.5 years after the initial damage or to much time has passed and will have no effect on the L5 nerve root damage?” Yes. I have found that there can be improvement even years later decompressing a nerve root.
3) “Other then stiffness what “new” pain fusion may bring after successfull recovery (6-12 months)”? In general, fusion makes the back better but not perfect. If the surgery is not performed correctly or the fusion does not fully fuse, continued pain can occur.