How do I obtain the value of another observable, without it affecting the parent stream's return value

Solution 1:

You can just map back to the original value:

of('result1')
  .pipe(
    switchMap((result) =>
      mockHttpCall().pipe(  
        tap((innerResult) => console.log('innerResult: ', innerResult)),
        map(() => result)
      )
    )
  )
  .subscribe(console.log)