Communicating microcontrollers over bell wire
Modern-day Ethernet, as you are correct in suspecting, is designed for structured networks, not hanging random devices off random runs of cable -- the XBaseT family of physical layers are point-to-point, with all the collision detection done in a central hub IC (or not, if you have a switched network, which is the norm these days).  Given that you aren't trying to go all that fast, however, other physical layers designed for multidrop applications are much more practical for your application.

Two in particular stand out from the rest: the Controller Area Network (CAN) and RS-485.  They use differential data transmission (somewhat like modern Ethernet), but are multidrop (unlike twisted-pair Ethernet), and can support modest data rates (100kBits/s to 1MBit/s or more) over long cables (30m is no sweat for either one).

However, RS-485 is only a physical layer (transceiver specification) that leaves users to provide their own data-link layer (such as a special UART with some extra features), while CAN provides the data-link layer as well -- a wide variety of microcontrollers can be found with onboard CAN support (everything from ATmega parts such as the AT90CAN32 related to the Arduino's ATmega328 up to beefy 32-bit ARMs using Cortex-M4 or M7 cores, as well as automotive Systems on Chip with integrated CAN controllers), and standalone CAN controller ICs are available as well (the MCP2515 is one that is designed for simple microcontroller interfacing).

Furthermore, IP over CAN is possible and documented, and CAN cards are available for PCs (including Linux support, although it looks like if you want to do IP over CAN on Linux, you'll have to do a bit of kernel plumbing).

Last but not least, CAN generally is done using 3 wires -- two differential data lines, and a common ground to all nodes.  Isolation is accomplished at each node using optocouplers or digital isolators between the CAN transceiver and the CAN controller.  You can use the fourth wire to run say 12 or 24V power if desired, although I wouldn't run more than an amp over it.