NERVOUS

This is what is great about mat board, time is invested but not a lot of money.  Not discounting your time, but miniatures do take time and if you don't like what you did you can do it over without spending a lot more money.
Yes, applying a coat of varnish is fine for the sink and at least the edges of the mat board.  Go back and re-read the instructions I address the coating of the edges.  Take a breath and just go back and read, go step by step and slow down.  It's exciting to see this happen, you are creating your own miniature, something to be proud of!
With a blog name like yours I would think you are very well acquainted with chabby chic.  Do you keep note books on things you like?  I do, I go over them all of the time.  Go back and pick something.  You can always re-paint the cabinets.
This is supposed to be fun.  Funny I tell you all that, I would over think, over discuss (with any family member that was in ear shot) worry and just plain get mad over my scenes.  My family would learn to find ways to avoid me during these times.  They would tell me, "I thought this was your hobby and it was supposed to be fun."  "Blah, blah, blah, just tell me what you think!"  They are so happy now that I just make furniture and write the blog.
Really, just relax, it's just mat board.  TTYL  Kris