For point 1, if you want to use the railway approach instead of exceptions, you must annotate all functions in the chain.
For point 2, I was talking about the runtime cost.
For point 3, Result<Result<Foo,A>,B> isn't the same type as Result<Result<Foo,B>,A>. People in the Haskell world acknowledge that it's a big problem ("monads don't compose"), hence all the research on monad transformers and extensible effects etc.
For point 4, if your function is the lowest in the call stack and decides to report an error using the railway approach, you don't get a stack trace.
Foo -> Result<Foo1,Error>
Foo1 -> Result<Foo2,Error>
Foo2 -> Result<Foo3,Error>
I think the confusing part is the word "exception" as it mixes concepts.