Smart switch wiring with no neutral
The problem is, you're trying to solve this problem as a box i.e. you want to do all your improvement in this box.  But you have 3-way switches.  You need to solve each 3-way switch group as a group. 

Indeed, the right 3-way is totally separate from the others, and should not be cross-connected.  If your box allowed separators you could even put a separator in there.  

Rightmost 3-way

On the rightmost 3-way, this is location is the "remote" and both power and lamp cables come into the other end.  That is where the smart-switch should go.  Lutron Caseta all operates wireless, and their remote switches don't even use wires, so if you put the remote here you simply won't use the 3-way cable at all.   

If you want to use a mechanical switch, then you'll need to follow their guidance for wiring one, which is fairly illogical actually. What it's trying to say is you connect a remote switch between the Caseta's black wire and its blue wire.  If the switch is thrown, Caseta treats that as an on/off switch command.  

So in this case, the Caseta at the far end would send always-hot on the black wire to this location.  That goes to the black screw on the 3-way.  From a brass screw goes the red wire, back to the Caseta's blue wire.  White is unused.  The other brass screw is unused.

The rest of it

The left half of the box is complicated.  The top cable goes to a lamp, its wires are switched-hot (black) and neutral (white).  

The top left cable has 2 3-way travelers (white and red) and always-hot (black) heading off to a remote 3-way switch. 

The bottom left cable is kind of interesting.  It goes to the lamp controlled by that 3-way (that's the red wire), and it is also the source of power for the box (black=always-hot, white=neutral). 

Anyway, here, you would install the Lutron Caseta master, adding its always-hot to all the wires in the black bundle, and adding its neutral to the neutral bundle.  The "Load" would go to the red wire from the lower left cable.  

As it happens, the remote 3-way is already set up in an advantageous way, with red and white going to travelers and black going to common and that's already wired to always-hot.  So leave the black where it is.  Tap red for the Caseta's blue wire. The white traveler is capped off.