Jaggers calls Wemmick the most cunning imposter in all of London because <em>Jaggers disagrees with what Pip said</em> that Wemmick is a kind-hearted person. Because the <em>Wemmick that Jaggers knew is different from what Pip described him to be.</em> The Wemmick that Jaggers knew is different. The Wemmick that he knew can often be such a calculating individual in his business dealings. That is why Jaggers jokingly referred to him as the most cunning imposter in all of London.
It is false that reading a play rather than seeing it takes away all of its entertainment value. Both reading and seeing a play can be very entertaining, it has nothing to do with the medium.
You should chose Grandmother