How do I make sure I keep my benefits when switching to relief work? I have worked over 520 hours in the past 6 months.

You will need to keep working at least 520 hours in each 6 month period--periods are defined as Dec-May and June-November. Each December and June they'll evaluate whether you met that obligation to have insurance for the next 6 months. (Article 15.1.2 of the contract).

flex hours counting toward benefit eligibility as i transition to relief staff?

This is what the contract states about relief staff and benefits:

 

15.1.2    Relief employees.  Relief employees are eligible for insurance coverage if they have been compensated for at least the hours equivalent to .5 FTE during a qualifying quarter.  Insurance coverage for such employees shall begin on the first day of the calendar quarter (January 1, April 1, July 1 or October 1) following the qualifying quarter.  Qualifying quarters are defined as:  #1 December through February (April 1st coverage start), #2 March through May (July 1st coverage start), #3 June through August (October 1st coverage start), and #4 September through November (January 1st coverage start).  The amount of the Employer’s insurance contribution to a relief employee’s benefit coverage is determined by the hours compensated during the qualifying quarter, consistent with the provisions of Section 15.2.