After looking closely at your pictures I think you may be
perfectly correct. I understand what you are saying about a
hobby project - been there, done that many times myself. Just be
very careful. That said, there are several things you can try.
The brown (looks brown in picture) wire is probably the hot wire.
If you have a meter or a non-contact indicator you can check
this. With the switch off, check for voltage between the white
wires (neutral) and the brown wire and no voltage on the other two
wires. A non-contact indicator is the safest since there is no
direct contact. However they are often so sensitive that it is
hard or impossible to tell which wires are hot and which are not.
If it indicates voltage on all wires try moving the tip away from
the switch along the wire and see if the indicator goes out. The
hot wire will always give an indication. The National Electrical
Code (NEC) will not allow two wires to be put under the same screw
or lug. This is probably why the contractor put one under the
screw and the other in the hole. You can try this also: Turn the
switch on, turn the breaker for this circuit OFF. Remove the
orange wire from the switch and tape it. Turn the breaker on and
check to see if some lights are not working. That will prove that
the orange wire is indeed a load wire going to the non-working
lights. The proper way to connect these wires would be as you
said, orange & yellow connected to a third wire going to the
switch. As to the colors, I suspect they were what the contractor
had on hand. The NEC allows any color to be hot except white,
gray, or green. Again, please be careful. Never use both hands
at the same time working on a live circuit. keep one in your
pocket. Rubber soled shoes, dry floor, etc.
I do not know much about Zwave. It may require a separate ground
for proper operation. There is a difference between neutral and
ground. The neutral is indeed tied to ground in most breaker
panels and always at the service entrance. The neutral (white)
wire is a current carrying conductor. It is the return path for
the current. The ground (green) wire is for safety and never
carries any current in normal use. In older houses it is common
practice (not good practice) to tie the neutral and ground
terminals together on switches and outlets. Most things will work
that way, however, some will not. Zwave should be able to answer
I hope this is helpful.