The key path literal you’ve used in the example will be typed as Sendable. It takes some effort to get a non-sendable one - it needs to be a subscript key path, capturing a non-Sendable index value.
Even if you get a non-Sendable key path, I think indeed if you don’t leave @MainActor (or any other actor), it should be fine.
But if you want to round-trip it, then indeed it is not possible. But that is not a keypath-specific issue, this is a general limitation of the
Sendable. Safe round-trip requires much more advanced type system features - see Preventing data races using dependent types for example.