You definitely need to take the assembly out so you can securely hold it while you pound in the H-Fit. Well, not pound it in with a hammer but more so with a mallet or something to soften the blow of the hammer. I told you it was a pain.
I'm also guessing your hitting in 1st and 3rd might be because it's not fully in either.
The instructions told ya to get the metal clip off of cable B. I had zero trouble with that one so... I dunno where you're stuck. Maybe calling H-Fit may enlighten the matter.