Great Tit in box fledging doubts and owl attacks

Have been following (no box camera tho) progress of great tits in my garden nest box. Yesterday was the 18th-19th day and the parents were still feeding. I then saw the female go to the box with some food, look in without going in, and fly away with the food.
I wonder if this mean they all fledged and she saw the nest empty, or could they all be dead in there? The parents did not come back after that episode. No more activity near the box.

In the previous week I noticed a little owl tried to attack the box, from the front and from the top. Attack failed as I also shooed it away.
This morning, early, getting up I saw the owl again, saw it take off in a hurry from a bush near the box. It was so quick I could not see if it had prey.

Really would like to know if it is likely that the chicks would die so late in development. I have been looking at the nest all the time and really hoped to see them fledge this year.
And how common is it for a little how to try and predate great tits in their nest? During the day?
Thank you

Alex