Have you taken into account the distinct possibility that what was observed was not a "head on" collision, but that the objects orbited each other; in that case the last seconds would have given rise to fluctuating waves, rather than a steady increase.
like I said in an earlier post , if the two black holes
lined up with the earth as in an orbit around each other
like the planetary alignments seen in our solar system then
the planetary alignment should cause a gravity fluctuation that should be seen from outside of our solar system.
likewise if two black holes lined up there should also be
a fluctuation seen from outside the two black hole system
as the black holes orbit each other.
you would see and measure both a increase and decrease in
It has been estimated that in the final seconds, objects such as two massive black holes could be orbiting at about 100 times per Sec. and travelling close to the speed of light. In that sort of scenario, a lot can happen in fifteen Secs.
if the two were orbiting at 100 times per second then
the increase detected would have been at 100 iterations
per second and steadily increasing with each iteration
until the two merged , after which there would be a final
and observable increase.
the graphs describe more of a passing of the two masses
than a merger.
slow approach with steady increase in intensity
followed by a faster departure with a steady decrease in
intensity then the graph shows a return to normal intensity.
they did a slingshot , judging from the faster departure
shown on the graph.
I could understand two grains of sand orbiting at 100 times
per second , but not at a angular velocity close to the speed
of light in a vacuum.
personally and logically I don't know that two massive masses
could be held in an orbit by any force if they are traveling
at such an enormous rate of angular velocity.
logic tells me without even breaking out my calculator
that the centrifugal forces would greatly overcome any
gravity field that the two masses could produce combined
and the orbit would be broken.
they would then go their separate ways.
now if the two black holes simply collide head on then
sure this would stick them together but there would have been
a distinct increase seen after the two merged not a increase
followed by a decrease to the previous gravity intensity as
depicted on the graphs.