As a result the failure was within limits for certification and almost 50% more than what you refer to as the "zero margin".
You are confusing "factor of safety" with "margin"
Factor of safety is the relation between ultimate load and limit load, generally a 1.5 (or 150%) is required here. Some special factors may apply.
Margin is between rupture load and ultimate load.
Zero margin means rupture at ultimate load, or at 150% limit load.
are flight control movements tested at the same time? eg, the wing is stressed to 125% of its design limit, are all flight controls then tested through their full travel to ensure full control if the wing sees such loading in flight
No.
Everything tested above limit load is not "seen in flight" (at least it should not).
The rule requires flight testing of the control system up to 80% limit load and ground testing (or analysis) of the control system up to limit maneurvre load.
Nothing above limit load is tested in combination with other loadcases or other systems.
If in doubt, read the rule. It´s available on the web.
i.e. CS-25
CS 25.305 Strength and deformation
(a) The structure must be able to support
limit
loads without detrimental permanent deformation.
At any load up to limit loads, the deformation may
not interfere with safe operation.
(b) The structure must be able to support
ultimate loads without failure for at least 3 seconds.
However, when proof of strength is shown by
dynamic tests simulating actual load conditions, the
3-second limit does not apply. Static tests conducted
to ultimate load must include the ultimate deflections
and ultimate deformation induced by the loading.
When analytical methods are used to show
compliance with the ultimate load strength
requirements, it must be shown that –
(1) The effects of deformation are not
significant;
2) The deformations involved are fully
accounted for in the analysis; or
(3) The methods and assumptions used
are sufficient to cover the effects of these
deformations.
CS 25.683 Operation tests
(a) It must be shown by operation tests that
when portions of the control system subject to pilot
effort loads are loaded to
80% of the limit load specified for the system and the powered portions
of the control system are loaded to the maximum
load expected in normal operation, the system is
free from –
(1) Jamming;
(2) Excessive friction; and
(3) Excessive deflection.
(b) It must be shown by analysis and, where
necessary, by tests that in the presence of
deflections of the aeroplane structure due to the
separate application of pitch, roll and yaw limit
manoeuvre loads, the control system, when loaded
to obtain these
limit loads and operated within its
operational range of deflections can be exercised
about all control axes and remain free from-
(1) Jamming;
(2) Excessive friction;
(3) Disconnection, and
(4) Any form of permanent damage.