Valerie- you have so many great questions!  I am glad you are starting another Olga's Vest!  I will definitely post KAL updates in the group, but it does feed at the top of the group page as well... or at least it should.

It gets tricky to heck gauge in pattern, especially here where there are 21 sts to 4" that means you need to get 5.25 sts per inch.  For a lot of people this is very tricky to check and some yarns are harder to read that others.  I would cast on 21 sts and work for a while and see if your swatch is 4" that is the truest way to check gauge in a stitch pattern like this.  Don't concern yourself too much with checking every inch.  

I suggest a 4" swatch to see if you get gauge... if you do and are still not sure you like the drape, make a bigger swatch.  The pattern repeat for this garment it a multiple of 2+1 (this just means make sure you have an odd number of sts).  Make the swatch as big as you like to check drape.  

If you got gauge for the stitch pattern on a 10.5 this just means that you are a tighter knitter than me and when I suggest to go up to a size 10 (for example) for the bind off then you will want to go up one needle size from the size you got gauge.

If the buttonholes were too big, it might actually be that your buttons were too small.... what was the diameter of your buttons last time?

Stitch gauge is definitely more important that row gauge USUALLY.  In this particular pattern stitch gauge is the length of your garment and is very important here.  In some garments the pattern is written by rows rather than inches, so it's definitely a good idea to check your row gauge and see if you are getting that too.  Sometimes row gauge is harder to get, but it's also easier to work around.