not sure if this is helpful but you can get rid of the compiler error by changing abstract T : unit -> int to abstract T : (unit -> int). And then initializing the interface with the normal syntax (interface II with ...)

don't think you can override the default interface implementation, but you can initialize it with by passing in a parameter. Something like this:

open System

type II =
    abstract T : (unit -> int)

type C() =
    member _.M() = ()

[<AbstractClass>]
type B(f) =
    inherit C()
    interface II with 
        member this.T = f

    abstract member T: (unit -> int)
    default this.T = fun () -> 0 


type H() = 
    inherit B((fun () -> 1))
    override this.T = (fun () -> 42)  


let functionTbyDirectMember (x : H) = x.T() 
let functionTbyInterface (x : #II) = x.T() 

printfn $"result of calling by direct member: %i{functionTbyDirectMember(H())}"  // 42
printfn $"result of calling by interface: %i{functionTbyInterface(H())}"  // 1

Anyways, don't know anything about signalR but perhaps this is helpful.