there are nice tutorials for this. it's really easy.
i decided not to add new locations. the current location is fine. all cites you mention are totally overfilled with docks if you use BC. if i do something i think it through, i want it plausible and 'realistic' (in relation to a fantasy world). and the way the ports at the cities are designed is kind of stupid. the 'waterfront' is a farce, a sandbank. in a 'real' (fantasy) world a city like leyawiin would be built to CONTROL the shipping lane on the niben, not to block it. BC doesn't change this unfortunately... adding another port there would be even more stupid.
and yes, you missed something. read this thread carefully.

there will be NO TELEPORT / FAST TRAVEL for this mod. again: this is a sailing mod, not a portable house!
Okay. I completely agree with you about the position of cities, but mods to change things like that are far off. But I understand why you don't want to have multiple dock locations.
I think you misunderstood me about the Teleport thing though. A mod to connect the River Strid with the Niben would either have to cut through the un-detailed scenery of Valenwood and Elseweyr, or somehow go through the west weald. Going through the west weald would cause incompatibility issues up the wazoo, and it would be unrealistic to be able to send ships that way unless you make a locking canol system to go over the increasing altitude.(Which is obviously more work that it's worth, still prone to cause incompatibilities, and besides all those more important reasons, IMO beyond the technological level of the Setting) As far as I know, it is impossible to sail around the southern end of Tamriel. So, there is no way to get to the Niben, where most of the detailed coastline is in the game. Other mods fixed this by allowing teleport to a single location in the Abecean and a single location in Niben Bay. If you change this so that you have to be around a certain area in the Niben to go to the Abecean, and vice-versa, it simulates having taken the long trip around the continent.(I don't know what to say about jumping time ahead a week to a month or so, it would be realistic, but it would also cause serious issues with most all Survival mods.)
7. no. sorry. imo THIS is immersion breaking. in future version you will be able to hire someone and sleep while he sails to the destination harbor. i think this comes close enough?
I agree with you about Fast travel breaking immersion. In my game, I don't fast travel. I have it disabled with Ely's No Fast Travel plugin. What I'm talking about is having a way to get to the Niben, which has much more coastline to sail along than county Anvil. There are mods which deepen the water already out there, I believe they have been discussed already. What you have said here about hiring someone seems like a great Idea, but maybe not with the smaller vessels. (the two Elven boats and the small one from Siika's Dock resources.)