If lateral disc herniation can cause protective scoliosis in juveniles, can congenital small spinal canal at l4/5/s1 cause increased lordosis there?

It would actually do: The opposite-lead to less lordosis or even kyphosis as lordosis narrows the spinal canal ad kyphosis or bending forward opens the canal.

No: Increased lordosis can place increased stress on the posterior facet joints and this can lead to spinal stenosis. But the stenosis itself will usually cause a compensatory loss in lordosis. That is why you will see patients with spinal stenosis leaning over a shopping cart at the store. Leaning forward is a way to try to open up the nerve canal in a patient with spinal stenosis.