The reference layer will be adjustable (move/scale) in one of the next alpha updates.
In alpha 1 the default cursor when drawing or erasing is quite transparent and therefore had to see. Others have mentioned this too. We already fixed that, so in alpha 2 the cursor is more visible.
And already (alpha 1) you do have an option for changing the cursor to a dot (smaller but more visible). Did you try that?
Straight lines using a short cut like in PS is also on our todo for an alpha update. 🙂