First time root cutting of a 20 year old dwarf hawaii Umbrella bonsai on lava rock. Never been manage so its now about 12'' tall with three skinny long stems with leaves growing on top. I plan to cut all three stem down to about 6 tall.

Do I do flat cut or 45 degree?
Can I grow new roots on cut in water or soil without any root hormones? Water or soil will grow faster?
On cuts do I trim all bottom leaves and leaf two on top? Cut just leaves or cut it all way back to the stem?

Note this is a bonsai forum and not a houseplabt.propagation forum. Hawaian umbrella has not.much to do with bonsai. I would recommend going to a generic plant propagation forum