I had braces from 6th grade through the beginning of Freshman year, my grandpa and uncle were my orthodontists (helps to have "connections" to get your braces off the day before Freshman pictures)! I also had head gear, and an expander, to correct a cross bite like your daughter. I had 5 or 6 different generations of expanders from the time I was 3 years old up to the braces and then another during the time I had the braces on, too ( lucky it was in the family, or my parent's would've gone broke fixing my bite). With the cross bite it can take a long time to move all those bones to the right position, don't rush it. That said, if you really feel they are stringing you along, just go get a 2nd and/or 3rd opinion.
Talk to the orthodontist about your concerns that it keeps getting extended, remind him that he said 2 years and of how much longer it has been, and see if they can do the rest of the correction using the retainers she will be required to wear every night to keep things aligned, they can make minute shifts in teeth using them.
Good Luck, and make sure she wears her retainers AS PRESCRIBED, or else all this will be for naught, if you don't wear them, your teeth will try to move back to their natural positions, mine have shifted a little, even though I was religious with my retainers through HS and college - I haven't been in the last 10-12 years, and some of my teeth are slightly crooked again.
Jessie