I know it's a silly question (by which I mean anybody with the most basic knowledge of TES lore should know), but I need to incorporate this in the story of my mod, and I need to be certain. Would you take a ship from Leyawiin (or anywhere near it)?

I'd walk. Cyrodiil and Black Marsh are border provinces.

Yes, this.

Well, lets say that Tamriel is a lot bigger than seen in the games, where travel methods like ships, etc. actually make sense. Also, lets say you were looking to avoid walking through Blackwood.

Taking a ship from Leyawin tolany of the Port cities would be the safest, fastest and most reliable route I think. Otherwise it is walking through swamp/jungle/marsh and old decayed Imperial Highways which may or may not exist. It also might be easier to hitch a ride on some small boats through many different streams and rivers than to walk. If there is magical transport available that is always a bonus but the beet bet would be by ship from what I can tell.

It's a shame they didn't do more with Black Wood and the far south of Cyrodiil in Oblivion as it is supposed to be more of a jungle experience as it neighbours the three rainforest ( inc. marsh / swamp for Black Marsh) infested provinces.

I also wish the southern cities built on the Nibenay looked much grander as they were supposed to be the wealthy cities built on trade through their ports. Instead the majority of houses looked more like shacks expected in a shanty suburb or a the poorer shacks outside a city gates.