I'm not sure exactly how to solve your problem, but one thing that occurs to me is that lifetimes and references are really a compile time semantic. If you're dynamically loading something then you can't assert lifetimes, at least not safely. So for point 2 I feel like you'd need to create an unsafe layer in between the app and dynamically loaded plugins and then wrap that in a safe API.
For point 3 I suspect you'd need to look at the macro system. Some kind of like load_plugins!("~/.config/myapp/plugins") macro stuck in an appropriate place in the code that at compile time can check that folder and generate the appropriate glue code to register and use everything. One thing I'm not entirely sure about though is if you'd have permission to access folders outside of the project at compile time, there's a chance the compiler would refuse to do so, but I don't know enough about macros or the way they're sandboxed (or not sandboxed) to say for sure.