"Sajnos, en sietek" was not accepted. Is the "en" optional, or strictly never used in this context?
It is optional, only used for a special emphasis. Normally not needed.
"I don't know about you but I am in a hurry." - Here, I would use "én", because I am contrasting myself with you.
Miért »sajnos elfoglalt vagyok» nem megfelel?