Wow, I hadn't even checked the civ screen. The next year another goblin showed up as liaison. Looking at the civ screen, most everyone is goblin: Outpost liaison, diplomat, general, 13 of 16 barons/baroness' and the only duchess, but not the king.
There's also a goblin civ on the civ screen, which I suppose is because of the liaison. There is only a general and master.
Does this sound like one of the situations you're describing? There hasn't been a winter caravan yet, five years in, so I don't suppose there will be one at all.
Sure does sound like it. The likelyhood of them not sending any is because they have no compatible (as to say evil or first layer subterranean) creatures to use, not even for pack & draft animals (because goblins are sort of broken like that because they aren't perspective play priority)
Changing those circumstances can't be done once the world is generated but in the future if you add the relevant tags to the allotted evil creatures such as beak dogs that they are likely to use and encounter, they will be able to trade. (From a goblins pespective without wagons they cannot trade with their home nation)
'Master' is the law giver definition offered to demons who built and rule the site until they die of unnatural means (in which case a random goblin will take its place, in a constant succession battle on account of 'who's the biggest and baddest to not die a year in') I think a lot of the goblin nobility is generated from secret worldgen old-code on account of no 'generals' in the entity code and the fact that having a specifically titled goblin 'master position' is malleable, in which case goblins taking over the spot are 'lords' but still rank in as masters officially.
Additionally, demons/lords are responsible for setting world targets to attack, and generals are responsible for military 'strategy' which is really just the execution of arranging and moving armies to wherever their master points. Together from the 'secret code' making the bare minimum of what is required of nobility tags to be offensive without being skulking.
Stepping away from your fortress and opening legends mode would offer some insight into what happened, it is also surprising that goblins did not outrightly destroy your civilisation alltogether and bothered to conquer it. Most arrangements like that i've played in mean that often my own dwarven outcasts just reform the civ outside of the overtook boundaries (unless you're playing a previous model and goblins received some sort of change, it used to be more common a few versions back.)
From the embark screen (by retiring your current site for a second) can you gleam any information from your neighbours screen? (if the goblins are not
-------------- then they are neutral and probably ruling over you, you can change modes by pressing tab to switch screens)