From your description it sounds like you have the intelligence to understand this stuff, but are simply lacking certain nuggets of information. You'll want to read up a lot on "switch loops" and particularly "3-way switches". It will all make sense pretty quickly, then.
Oh, and one more thing that's a bit harder to uncover: In America, wire colors do not have firm meanings. Green always means ground. White or gray always mean neutral, unless marked or in switch loops and messengers - the goal being to build the necessary circuits with readily available multi-wire cable even though it's the wrong color. "Hot" lines are everything else - including marked whites and (implied) switch-loop whites, and there is no official standard of how to designate certain types of hot. (however in commercial facilities, consistency is required.)
This is not as hard as it looks
We can tell from your photo that the far left cable provides the always-hot and neutral for the lighting branches on the left and center switches, which leave via the two middle cables, while the far right cable is for the 3-way circuit and thus needs to be left alone. Furthermore, we can see the neutral bundle for the two outdoor light circuits in the back of the box, running left to right.
This means that putting both outdoor lights on the same smart-switch is easy. The neutral bundle gets pulled out from the back of the box and the switch's neutral gets added to it, with a pigtail of appropriate gauge wire used to make the connection. Likewise, the existing ground bundle is pigtailed to the ground screw on the switch, and another pigtail is run from that ground bundle to the ground screw on the 3-way switch to the right, as the grounding in this box appears to be improper.
Then, we can cut the wire that connected to both the left and center switches (on their lower screws in your photo) back to just before the first stripped section, strip an end, and connect it to the LINE terminal on the switch. The two wires going off to the outdoor lights are then wirenutted to a pigtail that connects to the LOAD terminal on the switch. Get a suitable faceplate (3-gang with a blank in one position, a decorator or toggle in the second, and a toggle opening in the third -- modular faceplates are also an option for this, or you could use a switch-filler in the existing faceplate if your new smart switch uses a toggle style handle), button it all back up, turn the breaker on, and enjoy your new smart switch!
Best Answer
No, you cannot bootleg neutral from the very rickety ground BX provides.
It worked for 80 years because normal switches don't need neutral. You are introducing a new wrinkle: a smart switch that does. Here is how classical 3-way circuits work. Remember current flows in loops, and the route may differ, certainly as will the color codes.
Note that neither 3-way switch uses neutral, and the "remote" 3-way doesn't even have neutral anywhere near. Remember: installers are stuck with the colors that cables are manufactured with, so white wires may not be be neutral. Travelers are the ones that land on brass screws, they can be any color. I prefer to mark wires with colored tape to indicate their actual function, but apparently, I'm weird.
The problem is you want to put the smart switch somewhere it can't go. You need to rethink.
Generally, you handle this by carefully selecting a smart-switch master that goes at the switch location that does have a neutral. Then you either accept a plain switch at the remote location, or use a matching smart switch "remote". In the latter case, you re-task the 3 wires to be whatever you need for the remote to work.