kevkev,
You asked about where a kid with Ivy interest was seen...
My 2012 son had "offers" from many of the Ivy's. Things have changed a little bit since then, as several schools (Brown, Cornell, Penn, Yale) were doing showcases with a couple other primarily academic schools and called their showcase "GPA Baseball". Son was seen by those schools there.
He also was offered by two Ivy's after the PG National Academic Showcase. Ditto two more at the Headfirst Honor Roll Camp (now I see one or more of the GPA Showcase schools resuming their participation in Headfirst).
The Princeton Camp was referenced to us as very good and included participation from neighboring schools. But we did not attend that. Typically late July or early August timeframe.
Son did the February Harvard bubble camp (quite a story in its own right) and there were a lot of neighboring schools attending that too.
All the Ivy's do the prospect camp thing, sometimes more than one time per year. But we found it was not necessary to attend one of those to be offered. However, we ultimately visited most of the campuses at some point in the journey.