Gannet Fan,
I'm 6'4"...
that presents a problem with the anthropometrics as seating is not designed for the ends of the distribution curve.
I have no chance of adopting the brace position
if the seat in front is unoccupied, there is time, and it has a break forward feature ... push the back down and then use the normal crash position. If that is not applicable, I suggest one would opt for maximum tightening of the seat belt assembly and still adopt the normal position. The seat back in front will have some degree of delethalisation padding and, for modern aircraft using dynamic standards seats, the HIC will have considered head impact on the seat during the certification program. Main thing is to have the belt tight and rely on the upper body restraint keeping the head impact impact velocity down.
Can I assume in a real life situation of a crash that my seat will come adrift from its mountings
For older designs this is a problem. The seat and seat/track design is based on static loads and any seat track disruption can precipitate separation .. so, the answer for such aircraft is that it all depends .. especially on the dynamics of the crash.
For newer designs certificated to the dynamic standards, the situation is far better with both seat and seat/track design so the risks reduce.
CAMI did some interesting tests in the early days of dynamic certification and demonstrated that a given seat performs dramatically better the larger the size of the aircraft .. largely due to the increased crush volumes underneath and in front of the seat ..
Rule of thumb ? Fly widebodies, not helos (which have the worst impact environments), fly dynamic certificated seats, preferably in aircraft of compatible certification. Another consideration is that the history stats suggest that the aft seats in the cabin generally have a better outcome in accidents.
... and, if that doesn't give you comfort, take note that the comparative transport mode accident stats still put scheduled aircraft above most other modes for outcomes. It always amused me that rail came out very well represented in the stats ...
... give me a good sleeper carriage or Mr P&O ...