Jesus Manu, it's soon 8 pages of dancing around a trivial issue.
Implicit casting from mutable to shared is unsafe, case closed. Explicit cast from mutable to unsafe, on the other hand:- is an assertion (on programmer's behalf) that this instance is indeed unique
- is self-documenting- is greppable (especially if implemented as assumeShared or other descriptive name).
I don't understand why are you so fixated on this. The other parts of your proposal are much more important.
