Seniority places you on a list by date of hire. Longevity is basically used for pay and benefits (i.e. vacation accrual, anniversary pay increase, etc.) purposes.

Example: John and Pete get hired same day. Pete takes 1 month of FMLA (Family Leave). 1 Year after their date of hire, John gets 2nd year pay while Pete remains on 1st year pay until 1 month passes (because he took FMLA). Then he gets 2nd year pay.

To answer your second question:
John and Pete are both put on the "global" (master list with all pilots on it) seniority list when hired. John may have taken the upgrade first, but when Pete takes it, he will be transferred from the FO seniority list to the CA seniority list. His global seniority number determines where on the CA seniority list he will be placed.

Hope this answered your questions.