I'm a tough Grader.
You gained marks in the first half, then lost them in the second half!
And to continue with your serious question, I can't see how the rock could NOT be from the floor of Gusev, unless it is a meteorite.
Of course, if the lake people are right, it could have been transported into the crater from a long way upstream, but I think the wind is turning against that model as fast as data can be acquired. Looks like the putative lake bed is a long way downsection.
And now, I suggest we let this thread die and shuffle off the page, and introduce any new thoughts as a new topic...