There used to be a hack for getting integrated windows using Remote Desktop, but I can't remember the name of it anymore and Google isn't finding much :( Hopefully someone remembers (and it's still maintained).
I do almost all my computing through Xpra these days. Being able to combine windows seamlessly from multiple VMs is a much more usable way to segregate workloads, and Xpra doesn't suffer from the same security (and increasingly compatibility) issues of X forwarding.
The currently shipping affordable RISC-V SBCs tend to be RISC-V cores embedded with a bunch of frustrating undocumented peripherals, more or less the same as with affordable ARM SBCs. It's disappointing :(
"We will just use technology to bypass this bad law" is a very convenient position for a technology-focused group to take. It feels weirdly privileged to just say "ah we will abdicate ourselves from the problem by using ever more complicated technology" rather than working to improve the legislation to benefit everyone.
Google's nsjail (https://github.com/google/nsjail) has a nice "inetd style" mode where it can launch a sandboxed process in response to a TCP connection for similar use cases to this (and is relatively quick to fire up).
It's basically like you described - an informally specified set of rules the various "big players" in e-mail enforce. There's some consensus about the things you "have" to do to participate in e-mail but it's generally not a black and white "do these things and you'll be fine" situation.
Edit: Found it: https://github.com/rdesktop/seamlessrdp. Seems like there are probably more modern solutions now though.