The returning value must be on the last line. You can't return the value in the middle code. To workaround this, you can create a var, set the value, and return it in the final line.
All expression controls are supported. You should have no issues linking any of the supported properties to an expression control.
functions like linear, velocityAtTime and wiggle are not supported. Probably some will in the near future. You can set your own function inside the code.