Actually you CAN get a take-off warning in the air. It occurs if the ground spoiler interlock valve is still open with the a/c in the air and flaps up.
I don't know at what height the a/c would be clean but I would have thought it would be a far bit before 10000ft (cabin px at which the cab alt horn sounds)
For me the give away as to the reason for the horn, is the point at which the horn first sounded. 10000ft is not high enough to couse hypoxia and reasoned thought should have deduced the cause. However. The fact that so many holes in the cheese lined up staggers me.
Incorrect sw pos after maintenance
Incorrect pre-flight checks (surely the elevation must be set on the controller and this is right next to the auto/man switch), after t.o checks, etc.
Poor (appalling) CRM
Cabin crew not aware of emergency access code to cockpit (or scared to use it?)
There were at least 5 layers where it all lined up.
I also have worked with one of the engineers involved and if he is one of those to be prosecuted then that will be a sad state of affairs. We need to learn from this and prosecution will drive untold nails into the no blame culture coffin.