I'm not so much into hockey.
Why is the fact that he didn't play for Boston speaking against a guest appearance?

I'm still waiting for a guess appearance of Wayne Gretzky. Maybe as ring bear(er)…

I'd say yes.
Look at Barney's blog from last week:
"“The Pineapple Incident” – One day we’ll let him in on this one."
http://www.cbs.com/shows/ho…

They could do so by telling Ted the story behind the Pineapple-Incident, which Barney, revealing in his blog, seems to know.

Maybe they didn't move to Rome? Maybe they just spent their holidays there?

I think the writers want us to believe it was Ranjit. But I think it was The Captain, did thought that after Unpause and even more now. He might have offered Lily a different job where both dreams could be lived