Electronic – Should I use Magnet Wire as a conductor in this experiment

I think it may be something like FFC/FPC cable, which is often used nowadays for PCB connectivity.
It's not the easiest stuff to hack being very delicate. Some is made a bit tougher and intended for e.g. hot bar soldering but I think not the type shown in the photo.

Is it flexible?

You may be able to make a connector/switch from a couple of bits pf PCB, depending on the tools available to you. I have etched an connector a few times for soldering display FPC connectors to, but it's not the easiest and this is a bit different anyway as mentioned above. You want something that clamps onto the contacts.

For more of the cable, look on Digikey/Farnell/Mouser/RS etc, for FPC and FFC cable (and connectors, and similar speakers (look at datasheets and they will usually say what connector to use). You may find something you can hack into what you need, or maybe even some ready rolled switch with the required connections (though I am not aware of any, will update if I find something)

I do think this will be very difficult to accomplish, but it's always fun to try :-)

The pitch (space between contacts) looks pretty small - what is it roughly? (hold a ruler with mm markings beside to judge)

A recommended approach would be to use an optocoupler followed by a comparator (eg. LM339), or better, an integrated part such as the Fairchild Semi FODM8071 logic gate output optocoupler.

The reason the optocoupler is recommended:

There is likely to be a ground potential difference over a 50 meter cable, also the possibility of picking up EMI over the long cable. The optocoupler eliminates any ground loop / potential mismatch concerns, as well as any need to precisely match the sensor's supply voltage to the microcontroller's.

The use of the opto will allow a higher voltage to be used for the sensor circuit, reducing EMI noise sensitivity.

An added benefit of the specific Fairchild part suggested above is its high noise immunity. This will result in a more stable signal acquisition, important given the distances involved.

FODM8071 is a 5-pin leaded SMT part, so using it is essentially like not having to build any additional circuit - you could wire the part and its few support discrete components up deadbug style, if you like, or put them together on a proto-board PCB.