As flaperons are not exactly consumables, I'm sure it will not be all that difficult to account for all which are not attached to an operating airframe.
I am not too familiar with the 777, but if you would find an Airbus rudder, I would not be too sure that it must belong to a crashed aircraft. A lot of them have been scrapped. Same applies to Spoilers on several type of airplane. The first generation of composite parts
have been consumables that were often found to be beyond economic repair...
Putting probabilities on it is pretty pointless.
unless probabilities are in the high 90% range
The point is: we need proof that this item is documented to be the last one installed on the MH370 airframe. We should not be happy that it "most probably is"