Thin, flat pressure switch
Consider a capacitive touch sensing solution instead of a mechanical contact switch if that meets the requirements:

For discrete "keys", pieces of single-sided PCB cut to the right shape and size serve well as capacitive electrodes. For sensing, either use a suitable microcontroller which supports capacitive sensing, or use a touch sensor controller such as the Atmel QTouch series. 

An illustration of how this works, from the Atmel QTouch pages:



The PCB material, either glass epoxy (FR4) or paper phenolic, serves as the dielectric in the diagram above. The single-sided copper layer would be on the inside of the wearable. 

From the question, it appears that a large number of "keys" may be needed. In such case, discrete capacitive touch sensing (one pin per electrode) may be sub-optimal. A matrix touch sensor IC would work better, e.g. Atmel's QMatrix. From the Atmel site: 


  QMatrix uses a pair of sensing electrodes for each channel. One is an emitting electrode into which a charge consisting of logic pulses is driven in burst mode. The other is a receive electrode that couples to the emitter via the overlying panel dielectric. When a finger touches the panel the field coupling is reduced, and touch is detected.


This works much like the matrix scanning technique used in conventional keyboards. 

For up to 48 buttons, the Atmel QT60486 can be used: Up to 48 sense channels, with data communication via SPI and UART. 

This capacitive touch approach addresses durability, safety and production concerns in one go. For best results, the exposed copper side of the electrodes could be coated with standard solder resist, or for longer life when exposed to human perspiration and friction, a suitable epoxy coating.



Addressing the bullet point questions:


Copper as on a PCB would work fine for either the capacitive touch solution in this answer, or on the mechanical switch in the question.
That would depend on the copper thickness, the pressure per button press, and any other source of mechanical wear. Of course, for the capacitive touch solution, life is indefinitely higher
You do need to be concerned about oxidation, especially if the actual contact area on the conducting surfaces will be small, as appears the case with the mechanical approach in the question. Again, not a concern for capacitive sensing.
High frequency low voltage would work best - no polarization-related deposit of airborne salts, and yet safe for a wearable. Not relevant for the capacitive solution.