Over the years this board has actually had some very helpful information for parents and girls interested in the ivy league schools. For reference, my daughter played 4 years of ivy volleyball and was actively recruited by 4 of the 8 schools and was accepted early decision.I must admit however that it is not clear to me how the schools that have eliminated early decision-Harvard and I think Princeton are now dealing with their athletic recruits, but I would guess that the "likely letter" you referred to is now serving a dual purpose.
1-Let me start with your last question about whether a borderline academic admit will struggle to such a degree that she will not enjoy herself. My sense and experience with my daughter's teammates and the other girls she knew who went ivy is that the schools are generally careful to make sure that the girls who are recruited and admitted are quite capable of handling the academic load at the particular school. Most of the girls are serious students and I have heard more than one team parent comment that they told their daughters- you have the choice to play volleyball, study or party-but you will only be able to do 2 of the 3 well. Most of the girls seem to choose the first 2 and seem to do fine academically.
2-Parameters of GPA and SAT scores. Each of the schools is different but generally the incoming recruits have to have an "academic index" in a particular ratio to the incoming class as a whole. The discussion of what is an academic index and how many standard variations from the overall class average is necessary is best left for you to google (in the days of the old 1600 point SAT it was a combination of the gpa and the 2 parts of the SAT's). Bottom line your gpa and SAT's probably have to be highest for Harvard Yale and Princeton -then Brown Columbia and Dartmouth-then Cornell and Penn. On the other hand the higher your daughter is on the coaches "list" that she is submitting to Admissions the lower her scores can be with the converse being true as well. A girl with a great gpa and terrific SAT's who may see little playing time can balance out the top recruit who may be borderline academically.Of course "borderline " academically at one of these schools can be pretty impressive nonetheless. Again using the old 1600 SAT numbers , I would guess anyone above 1300 would clearly be "in play". 1200 would probably require a better gpa and better hops and a stronger arm swing. Bottom line is this-call a few of the coaches- they all have a very clear understanding of what kind of numbers they can expect Admissions to ok.
3-What assurances can an ivy offer. Historically the ivies used early decision along with "likely letters" to compensate for the lack of letters of intent and the requirement that Admissions actually has to say yes to all recruits. Early admits were notified in December and generally were committed to attend that school. The likely letter was in reality an early "early decision" notification from the Admissions office that I think was designed to deal with girls who had scholarship offers from other non-ivy schools and did not want to pass up a scholarship without the comfort of knowing they were virtually assured of admission to the ivy of their choice.( A girl pretty much had to flunk a course or get arrested to blow a likely letter).By the way, the ivies have lots of money-will give a recruited athlete a "pre-read" on a financial aid application(as set forth above-no athletic scholarships- only need based) and if you get financial aid and your daughter decides she is done with volleyball, she still gets her scholarship and an ivy education.
4-I am unaware of anything official from an ivy school before the Fall of the senior year. However if the coach really wants your daughter,and the feeling is mutual, and it is abundantly clear to the coach that her academics will pass muster with Admissions I have heard of informal commitments being made but I think they probably violate an Ivy League rule and really can't be relied on since they do not come from Admissions. There have been some interesting stories in the NYTimes about some of these issues being raised in the context of men's basketball recruiting.
I hope this was at least marginally helpful and I (and I am sure others) am happy to try to answer additional questions. I will repeat the advice I was given years ago when my daughter first started thinking about an ivy-call the coaches. They are used to dealing with bright inquisitive parents and will be happy to talk to you.