I can't answer any of your questions since we don't have a blended family or non-custodial parents in our situation. I do want to tell you that your step-daughter, who is as you say, a high performing student, may do even better financially at a private school. My son, who is in his junior year in college, was awarded enough of a scholarship that we pay LESS for his college education than if he had one to a state university. The state universities don't give much merit money - they don't have to. We didn't qualify for needs based money, but we did apply to FAFSA because there is an amount that they loan on an unsubsidized basis to the student. Our deal with our son is that since it is in HIS name, HE is responsible for the debt. IF his semester grades are above a certain point, we will cover THAT semester for him. If his semester grades were to dip below it, then HE is on the hook for that semester.
(Just to let you know, and unsubsidized loan starts accruing interest the day the loan is made - not after he graduates. A subsidized loan will not accrue interest until the 4 years is done.)
The way we put it to him is that the carrot is that we cover his loan for good grades. The stick is that school is his JOB and it is his job to make the grades if he wants to leave undergraduate school with no debt. It's up to him, really.
I will also tell you that the private loans that you can get out there do NOT allow for a catastrophic event like the death of your child to get out of paying the debt. The government loans do. I would NOT take out a private loan because of that. The financial aid people at the schools will explain that to you if you ask.
Lastly, if I were you, I'd be looking at schools outside of your area who don't routinely use the CSS. You have several ladies here telling you that they didn't have this issue where their kids went to school. It's so much better to just change where your kids are looking than deal with issues that you're talking about.
Good luck~