Hi!
Ive never tried pure lava rock but my understanding is that it doesnt hold water well.
Ficus usually like moisture and tons of water so I would not place it under full sun except if its watered a couple time a day.
That is again theorical, more experienced members will chime in soon.
As for the cuts I would stop cutting if you want to get more mass, you have to let the secondary branching develop.
Cool project, I really like the placement with the statue!!
Have fun!
Ugo