Thanks for reaching out to us on HP Support Forums. 
I came across your post and would like to assist.
I understand that you have an HP Officejet Pro 8620 e-All-in-One Printer. I have read your post comprehensively.Thanks for the input.
However, in such scenario's where the admin password no longer works, you may perform a semi full reset the printer. Resetting the printer will restore the printer complete back to its original factory settings.
I have sent you a private message with the semi full reset steps. Also, you may access the complete user guide for the printer: http://hp.care/2p5huvA
Hope this helps. Let me know if you have any further questions. Cheers!