Yes, you can get the Wii Speak Channel ticket when you buy a used microphone

They are, however, nefariously penalizing third-party and home-built devices, preventing us from using headsets and anything not blessed by Nintendo.

I think that "nefariously penalizing" is a strong overstatement.

Nintendo are offering a (useless for many gamers) stand-alone chat option for anyone who buys the first-party accessory. That doesn't prevent third-party devices from being made or used, and it doesn't prevent you from buying or using a different solution.

Additionally, third-parties could (theoretically) develop their own stand-alone chat solution. They could distribute it either via disc that would install the channel on your Wii, or online through the Wii Shop Channel - although the latter option would probably require them to pay Nintendo for using the service. By using an online service similar to that provided by EA games, third-party chat options could even be much better and more flexible than Nintendo's own if they were willing to front the cash to develop it...

Of course, it's much easier to villianize Nintendo than to demand quality from third-parties.

What exactly prevents someone from requesting a ticket without owning the Wii Speak?

I'm assuming they'd ask for a unique serial number off the unit, to make sure you don't get a wiispeak channel for third party peripherals, and this brings up another question:

When you give them that serial number, will they inactivate old installs of the channel that used that particular unit? Let's say for example I've got two wiis, and want a second code for my second console. When I call Nintendo and give em the wiispeak unit's serial number, will they deactivate the wiispeak channel on my first wii before giving me a code for the second? Or if I tried getting a second code to use on a friend's wii, would they kill the channel on my wii before activating the code?

Just find a WAD of the channel and install it if you don't wanna shell out for an overpriced shitty headset. Nintendo isn't selling it anyway, even though those whom have other headsets might still want to use it.

I don't get why Nintendo has an obsession with making sure only people with WiiSpeak use the channel, period. This is something that should be built into the system menu and software, not packaged into a small one-off channel you're never going to use.