First time was 1988 and it doesn't say what the violation was for.
Then, from this article:
McStay murder mystery: Who is Chase Merritt?
"Merritt's most recent felony conviction was in 2001, court records showed, when he admitted to stealing $32,000 worth of welding and drilling equipment from San Gabriel Valley Ornamental Iron Works in Monrovia, Calif.
He pleaded guilty to burglary and grand theft charges and was sentenced to 180 days in jail.
Merritt violated probation several times in subsequent years and failed to appear in court on numerous occasions, resulting in his probation period being extended and at least seven warrants being issued for his arrest between 2001 and 2010, the records showed.
His probation from the 2001 felony case ended on Sept. 14, 2010."
As for when Chase officially became a "person of interest" in the murders? Well, he admitted to being the last person to see Joey. I would think he was a person of interest from the minute he said that.
Also, with disappearances and murders, it is the immediate family/circle of friends/business associates/enemies that are looked at first. Pretty sure that is textbook, no?