Sep IRA and health insurance premium and shareholder w-2 of S-Corp

Hi , Assume my reasonable Compensation or W-2  as sole owner of S-Corp for all year would be $85000. Let me know, are health insurance premium, SEP contribution a kind of most of fringe benefits included in $85000. If yes, net pay can be reduced when I get my paystub. 

Questions: 

If I want to do contribution through Fidelity, my max SEP contribute is 25% * 85000=$21250 so can I contribute gradually by each paystub?

Can I contribute in 2024 before March 15 for 2023 tax return in lumpsum from my personal checking account? or should pay directly from S-Corp checking if so but I confused about my paystub and my W-2 that dose not show anything for this contribution?

And what can I say to payroll provider to handle for this contribution which I am going to make in Fidelity, what box of my W-2 should be affected indicated for this contribution?

And what happens on federal tax withholding for box 2? Should I tell them, payroll provider.

I need to figure out step by step. I would greatly appreciated it!