1. Is the use of the word shew instead of show of any significance, or is it just a mistake?
It's a Britishism that shows up in Shaw for example (or "shews" up there, if you will). Ed Sullivan in the early days of television used to promise his viewers "a really big shew" but that was more a question of his accent than his orthography.
2. Of what significance is it that women are portrayed as lines (phallic symbols) and the ideal male as a circle?
You are right that there is an interesting reversal of Freudian archetypes here. But where was Freud during all this time?
3. Is Abbot's correlation between physical attributes and mental ability of Flatlanders a personal belief or a point of satire?
Abbott himself was somewhat short and not very athletic, so I don';t think he subsribed to a Grecian ideal physique as the determinant of intellectual power. Other people were more inclined to judge by appearances, and Abbott riduculed such suggestions. More serious scientists, such as Galton, spent a lot of time measuring prisoners` skull size, arm length, et cetera to try to determine "criminal types" so that may have been a specific referent in the treatment of configuration as a determinant of social status in Flatland. The debate still rages. It's Abbott, with two b's and two t's and two Abbott's as well, by the way.