Congratulations on your upcoming retirement! Let’s address your questions one by one:

1. Are healthcare premiums taken out pre-tax in retirement? No. All deductions from your MOSERS pension benefit are POST tax.
2. As an employee, my premiums are taken out pre-tax to lower my taxable income. Does this happen with a retiree's pension payment? No. All deductions from your MOSERS pension benefit are POST tax.  
3. If healthcare premiums are deducted prior to federal tax deductions (while in retirement) than the prepayment of healthcare premiums (at year-end) would be the same, is that correct? No. Healthcare premiums in retirement are NOT deducted prior to federal taxes. So, there may be tax advantage to you to pre-pay your retiree healthcare premiums for the remainder of the calendar year from pay you receive as an active employee. See the information below from the MCHCP website:

Employees whose premiums are collected pre-tax through the cafeteria plan have the opportunity to pre-pay premiums pre-tax as a retiree. Prepaid premiums may only be paid within the same calendar year*. To prepay retirees must submit their enrollment request at least 31 days prior to their retirement date. The first month’s premium for retiree coverage will be divided between the last two active paychecks. Additional prepaid premiums may be collected from the retiree’s last two active paychecks and/or lump sum vacation/compensatory time payroll. Verify this payroll amount with the HR/Payroll personnel to determine how many months of retiree premiums can be prepaid.

*Employees with a retirement date of Dec.1 cannot prepay premiums, since future premiums would be for a different plan year. Employees with a retirement date of Jan.1 can only prepay premiums using funds from their final active payroll (usually Jan.15) and/or their vacation and compensatory time payroll.