Learn Turkish: Toplantı zorunlu, bu yüzden kestirmeden gitmiş olmalı. - The meeting is mandatory, so he must have gone via the shortcut.

This structure combines the inferential past ending -miş (used for things we gather or deduce) with olmalı (must be). Literally, it is like saying 'he must be in the state of having gone.' We use this specific combination when making a logical deduction about something that already happened.

Could I say gitmeliydi instead of gitmiş olmalı here?

No, they mean two very different things! Gitmeliydi means 'he should have gone' (an unfulfilled obligation or recommendation in the past). Gitmiş olmalı, which is what we see here, means 'he must have gone' (a strong logical deduction that he actually went).

Why does kestirme (shortcut) take the -den ending? Doesn't that mean 'from'?

Yes, the ablative ending -den usually means 'from,' but in Turkish it is also used to mean 'via,' 'by way of,' or 'through' when talking about a route. So, kestirmeden gitmek means 'to go via the shortcut.' You will also see this with phrases like kapıdan girmek (to enter through the door).

Is there a verb missing in the phrase Toplantı zorunlu?