You don't really need to split 1 & 2, since F# can define .NET interfaces and abstract classes just fine.

For that matter, you don't even need the interfaces if you wouldn't have had them in a C#-only solution. Just define the class in F# and use it directly from C#.

You still need a separate assembly for F#, but that doesn't imply dependency injection - again, just reference it and use it.

I've identified a possible use case for F# in a preexisting product; I'm looking for the simplest way to integrate F#.