I don't know if there is something official out there on this or if it's just from everyone's experiences but...
1. How often do non qualified recruits take the test to try and qualify? From the past, I don't remember a time during the season that a recruit became qualified and passed the test. It's usually been before or after the season but I don't know how accurate that is. Also...
2. About what GPA does a non qualified recruit need to have for you to think they will qualify before they come to your school? In my experience, a lot of my recruits with a 2.5 and above usually end up qualifying.
3. Has a non qualified recruit you've signed ever not come to your school?

I have seen recruits qualify during the season. Cant say how often, but from time to time.

At DI, I have not myself seen a nonqualifier decide not to enroll since 3.0 launched. I think that I have seen some juco3 players who I think had signed but not enrolled elsewhere, but cant confirm

In D2, I have had one nonqualifier fail to show up. I put minimal effort into him and did not visit him.

I've had multiple ineligibles not show up at D2. Not an insane amount. But enough to know that it's not a fluke when it happens.

And players can become eligible at any point during the season, all the way thru the end of RS2. The best thing to, is give every ineligible a color dot during scouting/recruiting. By doing this, when they do become eligible, you get a notification