I assume in the last sentence, you meant #10 not #11. Just making sure. If I were to use that(#10 drawing), what zener value should I use?No, those two diodes go to the two power rails of the MPU. This ensures that the output pin voltage cannot get more than one diode drop higher or lower than the range between the power rails. Any voltage outside that range is then conducted by one diode or the other to the power rail itself, and away from the MPU output.
Placing a zener on the pin to protect (as in #10) is commonly done for input pins and I don't see anything wrong with it here. As long as the output is normally below the zener voltage, I don't see the zener doing any harm and it does offer protection.
I do not understand the objection to #10 from OBW0549. The zener in #12 would pop like a fuse if any significant voltage came along to challenge it. There's nothing to limit the current. Arrange as in #11, the 1kΩ resistor limits the current that the zener needs to sink to protect the MPU.
Boy, these things are difficult