Typically, you don't lose anything for leaves of absences less than 30 days. Over 30 days, it varies by benefit and your airline's rules. I strongly suggest you call your airline's HR and insurance department to get the exact answers to your questions.

At one airline, medical coverage continued for the first 3 months of military leave. After that, it stopped or you needed to write them and pay out of pocket to keep medical insurance. Upon return from military leave, if your insurance was stopped, the insurance was reinstated on your first day of "active status." What constituted active status? I don't know. First day back on the job? Who knows?

Call the HR department.