Thanks! Space is the 2U dark blue on the bottom on the left side (and one of the 1U dark blues on the right). Return is the bottom left grey key on the right side. Scroll lock is… Nonexistent since I never use it lol although I’ve been adding in a lot on my second layer so I could add that need be.
Finger operated right trackball in the middle has been very ergo for me, I used to have a symmetrical Kensington orbit which I liked well enough but it broke on me. I find tilting the trackball in the orientation of my arm works well enough to move my right hand between the right side and the middle.
I agree with your sentiment on thumb balls, I used to use one but found it worse for my hand overall and my thumb started hurting.