Well, apparently I was right... The tree I showed is legal and is contrasting the lecturer's answer.
By inserting the following keys in that order: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10
and then taking 6
out of the tree will create a valid B+tree of height > 1
and n<10
.
This contradicts the h>1 ==> n >= 0.5*l*(l+1)
rule in the answer...
After many tries and lots of bureaucracy the lecturer accepted my answer and I got the points :)
Thanks for the try @Jonathan Leffler...