Russ, with this description, I'm fairly certain that installing Win7 will NOT solve your problems. Your problem is that you are using multiple devices for audio and midi.
Your issue stems from the fact that you are using the Real-Tek for audio output while also having a USB enabled mixer (an audio-only sound device) set up as an output device as well.
Windows cannot use both simultaneously as audio output. You have to pick one.
That's one issue.
You need to decide whether or not you need to use ASIO. Recall that the main reason to use ASIO is for low-latency 'live' rendering of either VST or VSTi. There is likely not a directly usable ASIO driver for the Real-Tek on-mobo chipset. If you want to continue using the Real-Tek for primary audio output AND use ASIO, you will need to run ASIO4ALL which 'wraps' the native drivers for the Real-Tek for ASIO use. It generally works pretty well - I've done this for a work laptop where I wanted a DAW-on-a-stick and had to use the Real-Tek on-board.
If you want to use the USB mixer however, you have to ditch using the Real-Tek. Period.
There should be ASIO drivers for the mixer. You didn't mention what the mixer is.
In connection with this is that the mixer will not have any kind of MIDI driver, most likely.
I think if you look up the Soundfonts thread in the forum here, recently posted, you'll see most people are using VSTi like sfz+ or other to render midi inside of the PC, not Coyote. DXi is nearly dead as a protocol - not aware of any developers continuing to develop music based DXi plugins. In fact it's possible that MS ditched on DXi as well with Windows 10.
However, there is a recent thread here about Coyote and Win10:
http://www.pgmusic.com/forums/ubbthreads.php?ubb=showflat&Number=312868It's probably time to consider 'switching teams' to VSTi. After all, PGMusic started going the way of VST when they bundled in Amplitube a few years back.
Switching over takes a little bit of thought, but not too much.
Do you need to input any MIDI info from keyboards? If not, then the sfz+ VSTi works great for soundfont-based General Midi playback. You might also consider a KONTAKT based player or other for the same functionality but with much more depth and sample quality.
I don't believe there is a direct sample set from Native Instruments, but the default sample set that comes with the free Kontakt player probably has all of the sounds that you need for your song playback - but you will need to do some of your own mapping of GM patch numbers, to the patch numbers in the Kontakt player
http://www.native-instruments.com/en/products/komplete/samplers/kontakt-5-player/free-download/This guy has compiled a list of Kontakt libraries to make a GM sound set:
http://bigcatinstruments.blogspot.com/2014/08/gm-midi-instruments-for-kontakt.htmlIF you get used to using Kontakt player, you might get hooked on the quality and then you'll be looking at expanding your sonic palette with other libraries.
Another option is Sampletank CS.
Yet another option are sample players that come with Computer Music magazine. I think Synthmaster CM has a way to load soundfonts. Not sure about that however.