I must admit I had loads of fun with this game a few years ago but then lost track of it for a while. It had absolutely the best and elegant crafting system of all the games in the genre but I stopped playing because as a whole it felt contrived, non-organic at points and lacked overall gameflow polish. Unfortunately from what I gather the devs stopped improving the game at some point, suggesting players to rely on mods instead.

So my question is:
1. How finished is the final official version.
2. How resilient the modding community is? Can I expect the issues to be ironed out at some point?

Well I can say I bought the game from RoboBob, at the Gnomoria official site years ago.I would say the the game feels complete when you play it. It is balanced out decently well, enough dangers to keep the game interesting, easy to use interface and simple to learn game play.
I do not no of any issues that need ironed out really if you are not sure or want to research it check out gnomoria.com. It has an extensive forum and you can see the history of development. I hope that helps answer your questions.
I would recommend this game to anyone whether you are a dwarf fortress player or not. The game play is fun and perfect for casual and intense gamers alike.

Are all those characters (except the mobs) in the trailer other players or can you get merchants (similar to Terraria) and/or hire soldiers?

While it looks interesting, it lacks even a basic tutorial and the help is not always helpful. So, in the current state I would advise against buying it.