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I recovered from L4-L5 compression fracture, I wore a torso brace for 6 months (which stretches the spine out and leaves space for the bones to grow to proper size).
I dunno what your personal situation is, and can only tell you what I would do (not having pain or scoliosis, I can't really know) but I was impressed by the effects of the torso brace, which basically lock your shit in the vertical, and since its is connecting the mid back and the hips and chest, no movement is possible, and you can crank it straighter mm by mm. I ended up enjoying walking around 2 inches taller on average.
Anyway, fusion of the L4 and L5 isn't going to matter all that much to mobility. The lumbars aren't capable of much flexion and are there primarily to vertically support and stabilize the torso.
You probably should dyor on the subject as I am sure there are lots of people that have had it done, I know of a bunch of people who have had it done, some were from car accidents, some old age, and none of them seemed like they suffered much in the way of complications.
I'm assuming you've already tried physical therapy and posture discipline. Those two followed by brace period, and can't be overemphasized. I needed to bring all the abdominals and more subtle muscles under control, and the only way to do that is practicing constant attention.
hope this helps, good luck