My living soil in a fabric pot has compacted, sinking the plant 10cm.

I mean yea, you could top the soil, but it'll probably discourage vertical growth and slow down current growth, due to all the new soil the roots can take over. Mulch shouldn't be a problem, its just going to decompose if you put more soil on top. And about the nodes on your plant; I would always chop the first and second node off the bottom during later stages of veg. Their not going to perform very well later on and are probably not worth wasting energy on during flower. For me personally, I would wait out for her to gain more vertical height. To encourage vertical growth I would dim the lights and hang the lights a bit higher. If you're outdoor, there is really not much else to do to dramatically encourage vertical growth. In the end, its really just a plant and it eventually will grow in height if it has enough light, nutrients and water.