No roots are growing from the "scar", they have pushed from the adjacent tissue. That plant was rooted about one year ago and has flowered normally since then.
I use pumice exclusively for rooting cuttings, I find stuff that light will eventually float to the surface of the compost as I tend to leave my plants long time in the same pot (more out of lazyness than by conscious choice). Roots appeared in a month or so in late summer.
The base of my mix is lava rock, or scoria or whatever it is called in your part of the world. It has a density close to one, is very porous and the particles are very irregular in shape and texture. The other major ingredient is something I cannot put a name on: it's the product of an eroding basalt face near the place where I work. It's not gravel, it's not hardened clay and it's not soil, but drains fast and still holds some water. I mix the two at a 2:1 ratio and this is used with up to 25% of something else - garden soil, worm castings,
terra rossa, limestone chippings, coco coir, whatever I feel will be usefull.
Another important aspect of my mix is granulometry: most of it is in the 2-5mm bracket, with some fine "dust" (this is important for water and nutrient retention) and some larger chunks to create an uneven and airy consistency that will allow the roots to explore. But bear in mid that I live in a warm mediterranean climate with bone dry hot summers and cold(ish) very wet winters.
Bottom line: mineral mix, mandatory for most mexican and all fat-rooted species and variously complemented for all the others. It tends to work well for slow growers but not so much for fast growers like
Trichocerei, I am still learning how far can I push these to achieve fast, even and thick growth.
Hope this helps.