well, it probably is 'simple' if you're a musician who hunkers down and uses only BandInABox...but here's what i might try to do in an afternoon:
obviously it would be great win10 OS could handle these video/audio tasks seamlessly (maybe macOS does?); barring that, is there a program that would enable me to help the computer do that?
I saw your diagram. That's a very reasonable setup and very close to my setup as well.
Your setup is very similar to mine in that everything revolves around the interface. The biggest difference is that I don't use a mixer.
The whole key here is to get that interface working properly. Once you get that, everything else will iron itself out. Once you get the proper driver (ASIO) working with the interface and the OS, your musical tasks should be seamless and without hiccups. The big question, and I may have missed this in the other posts, is.... Is that interface compatible with the Windows 10 OS and the computer? Serious question because, having been in digital recording for many years, I have had gear become obsolete literally after I did an OS upgrade because the driver "kinda worked" with the new OS but not in a seamless manner like it did on the same computer with the older OS. So check the interface and make sure 100% that it, and it's drivers are 100% compatible and will function with W-10.
To assume that I only use BB would be a mistake. I use Band in a Box, Real band, and Sonar. I've used this setup for over a decade on a dedicated DAW that runs rock solid. My interface is a Focusrite FW Saffire. I have migrated this interface across a few computers from a Dell off the shelf lappy to the custom build DAW I currently use in my studio.
Everything comes back to the interface and it's driver. Be sure those are compatible with the OS. I also assume you're not upgrading an older relic of a computer from say Millenium to W-10 because you might also have hardware issues as well with older machines.