All good dude... happy to help:
USB (as a computer protocol) can definitely send midi data... but only if the gadget it's attached to lets it do this. From what I read, I think the yamaha thing only sent backup stuff via the USB... i.e. it wasn't setup to send / receive midi... it only acted like a usb SD card!!
I'd suggest buying a dedicated USB controller keyboard... Using a USB controller keyboard will save you from needing to use a port on your 2x2 midi interface don't worry too much about knobs and sliders if you just want to practice playing and learning... You'll cut out cost by doing this
MIDI THRU QUESTIONS:
The midi thru stuff isn't configured in your computer. It's configured in the synthesizer / sound device. a single midi port can send information on 16 different channels. In theory this means that a single port can control 16 different devices. But it's still only one port / one chord out! To pass on the information past the first synthesizer / device, MIDI protocol uses "thru" to allow information entering the first device to continue straight out of a "thru" port so you could plug devices in one after; from THRU to IN; THRU to IN; THRU to IN.... etc..
In the "old days" (recently) all MIDI equipment would have 3 ports: IN; THRU; OUT. In would receive info; thru would instantly send that info out again and on to the next synth; OUT would send information from the synth (e.g. patch data or knob / slider movements) to be recorded / sent somewhere.... But THRU ports were used less and less (in fact MIDI band width screws up when using a lot of thru stuff causing timing issues and errors... so in theory - thru is kind of cr@p); These days many / most synths only have IN / OUT (but the OUT could be configured to act as a THRU in the devices settings)... why? cause thru is cr@p (as I mentioned) AND it costs less (one less port)...
If you want to; then it's probably possible to configure the Mininova OUT as a THRU... then you could send information through it on a different midi channel into the Volca.
FINALLY:
Well hopefully that explains THRU ports ok? BUT... You may not need to do any of that!!
How many USB inputs do you have free on your computer? I did some reading for you, and your Microbrute AND Mininova can both send and receive MIDI from their USB ports. So if they both use a USB port for their midi connection; then you can now plug in you volca to the MIDI interface and you still have one port free for your next synth purchase!
Then inside a DAW (e.g. cubase/logic/ableton) you can sequence each instrument / record or play them via the software DAW. With a controller keyboard you could play all at once; each one individually or any combination...
Hopefully tat makes sense... there's a lot of info to soak up in this post!!