Yes, immutability is great for safety. But the copies you have to make to keep everything immutable extracts a price in copies and garbage collection.

Rust is advertised as having fearless concurrency. That's true, but not that important as concurrency is not that common. What's important to everyday programming is Rust provides fearless mutability. The fearless concurrency you get with that is just a bonus.

Fearless mutability provides Rust the same safety as a functional language in a without the speed or space cost. IMO, it's Rust's true secret sauce.

Similar. I mostly design my code around something like pipe and lifetime. The longer something needs to live the closer it is to the start of the program. If I need to mutate it, I take care that the actual mutation happens in one place, so I can differentiate between read and write access. For anything else, I clone and I update. It may not be efficient and you need to track memory usage, but logic is way far simple.