Ive always been ambidextrous since i was a kid (read: my hand writing looks like shit) so when I started boxing switch-hitting came fairly naturally to me. As such I often go southpaw when sparring southpaws just to fuck with them.
For me, one of the biggest advantages of going southpaw is footwork. It just seems a lot easier to pivot out to safety than it does when fighting orthodox. In my gym we had a guy who wasn't a fantastic boxer, but what he was, was a very tough cat with a huge gas tank so he'd apply constant pressure, given the nature of amateur bouts he did very well. I always had trouble keeping him off of me until i went southpaw and suddenly it was really easy, all i would do is jab, step back, jab and then swing my rear foot around and repeat. He ended up chasing me all day as opposed to the usual scenario of me brawling with him in a corner.
I had quite a few fights against southpaws early on, one thing I have noticed is it seems they will rarely slip to their right. I can't give any logical reason as to why this is, its just an observation. They will almost always use range or duck to the left to get out of the way of a left hook.
The shoulder question, I don't think it completely negates it, but the angles presented are such that yes, straight hands down the middle from a southpaw make it hard to use a shoulder roll properly. The Zab Judah vs Mayweather fight is a good example of this. Floyd kept eating left straights early on, and ended up switching to a high guard and trying to catch and parry Zab's shots.
As for becoming more slick, its just one of those things youve got to keep in your head and continually try to improve at. Especially when your shadow boxing, don't just do what most guys do which is stand still and throw punches like there isn't going to be someone firing back at them. Practice slipping, rolling, weaving, pulling back etc. And when you're throwing punches, try to think about the position your head is ending up in at the end of a punch. For example I know against an orthodox fighter if I land a right cross to his head, the only dangerous punch he can hit me with is a left a hook, so instead of waiting for him to throw it, I will just weave under off the right hand. Having cat like reflexes is great if you got it, but it takes a long time and a lot of sparring to develop that sort of instinctive response to punches, and more importantly - why rely on it if you don't have to? Defensive responsibility is a good term.
What helped me most when I was a kid was watching hours of defensive fighting highlights on youtube. Seriously, i'd get home from school and load those up before I'd run off to the gym and then watch them before i went to bed every night.