Your questions make perfect sense. I personally agree with slowing down a horse to do shoulder-in. That being said, shoulder-in can only be done from a collected walk to begin with, so if your horse is strung out, he won't be able to do it without slowing down anyway. Try this. Get your horse walking energeticly on contact. The contact should be such that when you halt, he should be chewing softly on the bit without you having to make any adjustment in rein length. Walk him on a circle or come around the short side of your ring as you were on a circle. A few strides before you get to the long side, start applying the cues for shoulder-in. It's easier to learn this way since your horse is already starting to bend in the direction you want. He probably will initially seem confused of what you're asking or get stuck. If that happens, don't throw your aids away, but do straighten him out in a controlled manner, re-circle and ask again. Maintain the contact with your reins, but keep the contact soft so he can use his body properly to do what your asking. You're basically putting a 3rd leg down between his two hind legs. It requires both strength and balance on his part, so be patient.
You said you get shoulder-in confused with hanches in. It pretty much comes down to which part you're isolating, hence the names. If it helps, shoulder-in involves the horse being relatively straight from poll to tail other than slight flexion at the poll. During haunches-in, everything stays parallel to the rail and only the haunches move in.
FYI, if your horse has a poor canter lead, shoulder lead can do wonders to strengthen the outside hind of the affected lead. It's a fabulous tool to develop any horse.