That only works for C++ code using C++20 modules (i.e. for approximately nothing). With textual includes, you need to be able to switch back and forth the edition within a single compilation unit.
It's not clear that modules alone will solve One Definition Rule issues that you're describing. It's actually more likely that programs will have different object files building against different Built Module Interfaces for the same module interface. Especially for widely used modules like the standard std one.
But! We'll be able to see all the extra parsing happen so in theory you could track down the incompatibilities and do something about them.
Modules are starting to come out. They have some growing pains, but they are now ready for early adopters and are looking like they will be good. I'm still in wait and see mode (I'm not an early adopter), but so far everything just looks like growing pains that will be solved and then they will take off.
At the current rate, we'll have full module support for all of the most popular C++ libraries sometime around Apr 7th, 2618.
I expect modules to follow a S curve of growth. Starting in about 2 years projects will start to adopt in mass, and over the next 5-10 years there will be fast growth and then (in about 12 years!) on a few stragglers will not use modules. They are not free to adopt but there appear to be a lot of long term savings from paying the price.
Mixing editions in a file happens in Rust with the macro system. You write a macro to generate code in your edition and the generation happens in the callers crate, no matter what edition it is.