Have you ever recruited an ineligible who didn't come back for their last year of eligibility (their 5th year at school)? If so, does this happen a lot? I'm a DII coach btw. 

Not personal experience but a friend of mine has had a few ineligibles and all of them came back.

Only in D1 could a player not use their last year of eligibility and only due to them not having a cumulative GPA of at least 2.8 (or if they go EE).  In D2, ineligibles will sit out their first year but then have 4 years of eligibility afterwards while at D3, they can play immediately upon arriving on campus in that first year (no different than any other 4 year recruit).