I believe what MuttersDarkly posted waaay upthread is key here and I make no apologies for quoting it more or less in it's entirety:
The most overwhelmingly important factor in terms of outcome was not the schools we went to. It was the stage we were at when my parents' marriage collapsed. On the second day of my O levels to be precise.
My ears pricked up at the albeit small and innocuous sounding mentions of your husband. Might be something, might be nothing. But the two of you don't sound on the same page. And as a family appear to building huge time and debt pressures that generally don't do wonders for the glue that holds a couple together.
I'd take a long, hard look and assess the extent to which as pair the current situation might be wearing down, or has the potential to wear down the bonds between you in the future. Becuase a move as your daughter goes into year 10, while imperfect as a solution, is a million times better than leaving a pot to boil dry till it explodes off the hob in a year or two's time.
And you do sound under a horrendous amount of strain.
We all screw up. You might both have made wrong choices along the way. But the real measure of us is how we rectify those mistakes. So I think you both need to forgive yourselves and find way out of this that takes this awful, growing pressure off your family
It sounds like you both made a mistake a few years ago when you had the choice to send your DD to the almost as good free school. But that point in the road is done and gone and there's no point in blaming anyone for it.
Your DD sounds imminently mature - she's already reaching out to someone at the alternative school, I'm sure she'll survive no matter what decision is made.
But if you're taking her out, you need to do it now, not hope for the best and wait for later, and both you and your DH need to be on board with the decision for all DCs. Not in a dramatic crisis talks at midnight sort of way, but by sitting down once the DCs are in bed, with your finances and a calculator and working out what is possible and if you can, avoiding grand sweeping statements that seem to be solutions but will take a long time and money to come to fruition.
I do feel for you. We have one DS and we are able to send him to private school because a) fees are lower in Scotland and b) my parents help with the costs. We both work and have a good income, but we'd have had to sacrifice a lot to make it happen if we'd have been lucky enough to have 2 or more DCs. My BFs DD's go to state school and they seem perfectly happy there. It's not the be all and end all provided there is good parent support. although it must feel like it right now.