I would suggest you to use Yamaha for both input and output. You can use "monitor out" section for connecting your desktop speakers, and you can find how to configure the setup according to your needs at the bottom of the manual (Rear panel USB section). Basically this mixer is stereo input and stereo output only audio interface (when using USB). All inputs are mixed into 2 (stereo) channels. You would have to route channel 9/10 (stereo channel) to receive audio via USB and send that to monitor out section. Use something like this to connect your 3.5mm speaker plug:
In this setup you can easily route guitar -> ch_x input -> amplifier emulation software -> ch_9/10 -> speakers. Also, all inputs are combo XLR jacks so you don't need 6.3mm to XLR cable at all. Just standard 6.3mm to 6.3mm guitar cable will work (just like you have in the diagram).