Yes, it's possible to bring characters from other campaigns to the Monastery. Go to the Kaineng docks and ask to be taken to the mainland. You'll get a warning stating there isn't much for outsiders to do, but go anyway. You'll land in Seitung Harbor. Follow the Saoshang Trail to the Monastery.
I believe you take the docks near the market place south of Kaineng
Yep. You run yourself to the Marketplace, take the docks exit. Talk to the - well, whoever the guy is - First Mate Xiang or something like that, I believe. You'll arrive at Seitung Harbour. Take the Saoshang Trail exit to get to Linnok Courtyard, and then run down the stairs to enter SJ.
Yes anyone from another campaign can enter shing jea island and explore it to the fullest. Only thing you won't get from shing jea are quests, as those are only available to canthan characters. You can also of course vanquish the missions and explorable areas there in HM if you wanted to as well as farm like you wanted to do in NM/HM.