I had to have a planned c-section for similar reasons. I haven’t done much physics since school, but when the obstetrician explained labour and physical forces to me to me, she was talking in terms of Newtons, which I didn’t completely understand … but understood enough to know I didn’t really want to do that to my poor little lumbar joints, they’ve been through enough already! The pressure and forces that go through the lower spine and pelvis during a normal labour are enormous. And with a L5-S1 fusion, you’ve got no possibility of movement in the joint during labour, so your spinal cord is at increased risk of compression due to the forces, compared to someone who doesn’t have lumbar fusion (or even if they only have a higher joint fused … as you already know the L5-S1 is literally the last bit of your spine and your pelvis!).
If a c-section is being actively recommended to you, it’s for a good reason - someone qualified, who’s familiar with your medical history, and knows the risks vs benefits, is making a judgment call. If you’re in the UK, the NHS don’t especially like doing planned c-sections unless it’s really the best option. They do them on maternal request but rarely suggest them, and certainly not without strong clinical evidence to support it as the best option for the individual patient.
I’d follow their advice, but if you’re not comfortable doing that and are fixed on trying an unassisted delivery, it might be smart to pack your hospital bag as though you were having a c-section (get the high rise disposable tena pants instead of normal rise, etc.!) because you have higher risk of ending up as an emergency c-section case.
Your midwife and obstetrics team won’t want to risk you coming to harm, and I imagine they would insist on emergency intervention if it started to look a bit scary. It’s in your best interests and your baby’s best interests that you have a good birth and recover well. A good birth and recovery for women like you and me, and PPs, is not going to be the same as women who haven’t had a serious history of lower spinal injury.
My c-section recovery was brilliant - must admit, I was also really worried beforehand, but I was discharged with a good amount of pain relief and had everything set up well at home because it was all planned ahead of time.
Good luck x