You are probably right in all your points and one could easily classify the idea of this wall as stupid or a plot-inconsistency (neither do I remember any plausible reason to be given in the movie). Yet I'd say its stupidity served exactly the purpose it had to serve in the context of the movie.
It showed how desperate and, yes, stupid those bureaucrats were that cancelled the Jaeger program just because of some backlashes, only to build a wall that every reasonably-thinking human would classify as outright useless. But they were not thinking reasonably, they faced nothing less but humanity's end. It might on the very first thought make at least a small bit of sense (and is an intuitive human reaction) to build a wall as a last line of defense and this desperate and uninspired idea was the only thing they could come up with after the Jaeger program failed in their eyes. In light of a hopeless situation and mankind's guaranteed downfall, even the most stupid ideas may present at least a small bit of hope.
So yes, the idea of this wall as defense against the Kaiju is rather implausible, but on the other hand I think that's exactly how it was intended to appear, to emphasize the desperation and lack of further ideas of humanity in the presence of the Kaiju danger. And it may also have served to strengthen the audience's reliance on the Jaeger's to be successful, seeing that they are humanity's only reasonable hope left.