namedy - hugs to you! I think there are almost always compromises when you build, things you just didn't see coming that are annoyingly imperfect. Please remember that you are the harshest critic of your own build at this point. Others will look at it and see an amazing space. You see everything that is wrong. It does feel very disheartening, but remember what you HAVE achieved and how different your space is now from what it was! (You wouldn't be doing an extension if you already had the perfect house for your needs)
I am sure your family won't mind things being less than finished. We went to the inlaws when building work wasn't quite done, and the only thing that made it difficult was the fact that they didn't adjust any of their plans - they kept trying to do the whole bells-and-whistles Christmas (which was impossible), and it became really stressful as a result. We just wanted to have some lovely family time, and chill out. Christmas really ought to be more about people and less about trimmings! That's what really matters, after all.
I have a host of similar issues, too - most significantly, the crossbar on my glazing in the garden room is positioned so that when I'm in the kitchen (where the floor is around 1m higher) it is directly in my line of sight. This is SO annoying since it means I can't see a view. It was entirely avoidable, too - but I didn't pick up on it from the plans. I really blame the architect for not noticing this. I do get the same view, uninterrupted, from the large kitchen window, but I am still annoyed about this.
Other little niggles are getting me down - window seals that aren't perfectly fitted, a window with a tiny chip in the glass. etc. I need to get the double glazing company back out, and hoping this doesn't freak the builder who I know is worried about paying the bill for them. It has to be right, though - I've paid £13k for glazing and I expect it to look bloody well perfect.
dust - I think changing the payment schedule sounds like a great idea. They ought to be OK with that, it's only fair with the setback you've had.
For those of us who are continuing into January, Christmas is a big psychological roadblock isn't it? I keep having to remind myself that it's ONLY a week and a bit, and that it will actually be a great chance to reclaim the space a little from builders, and feel like it's ours again. I am really, really hoping that they tidy up a bit more than then have done to date, because downstairs looks like a bomb has hit it. I might have to have a very clear expectation setting conversation tomrorow morning with the chief builder.
My kitchen is coming on 27 December, and I am basically going to be building all the cabinets between then and the 2nd January, when work starts again.