Thanks. Sadly, my ISP provides a static IP, so I'd be forced to use a VPN instead. Which I don't believe both the Wii U and Switch support. But I'll take my chances anyway.
You can set up an OpenVPN config on your router, and then assign the VPN connection rules to just your Wii U. So your Wii U can connect via a VPN while not touching the rest of the devices on your network.
I did a lot of research, and it seems that as long as you format the microSD as FAT32 with 32K blocks, it should work just fine. The microSD to USB adapter is mostly for convenience, and to avoid the slow read speeds of the Wii U's SD card reader port.
Yes, microSD will work on the Wii U, but only if you use an microSD to SD adapter. If you try using a microSD to USB adapter on the Wii U, it will not work for the homebrew process at all. It MUST be the SD Port.
I could be misunderstanding your intentions / game plan, but that's what it sounded like you were going to try to me.