By going through the summary you have given i feel you should take Cinnarizine
instead of betahistine
...and that too half an hour before your flight....Secondly it happens every time you fly or had only once is not clear.....so you can take a Decongestant tablet also in addition.
Get done a pure tone audiometry (hearing test) to support the diagnosis of inner ear infection
as told to you by doctors.
this kind of vertigo can be because of
infection,,,or vestibular disorder
of other cause.
if infection then its mostly viral then you need to take some medicines like acyclovir etc..
so its better get done hearing test and consult ENT specialist
and if this happens every time you fly---then avoid flying take vestibular rehabilitation
exercise and medicines written above.