The answer to question 1b on eruvin 56a is listed as Taurus (presumably Eglah). Taurus is a near-equatorial constellation. The other possibility listed in "Background" is the constellation of Agalah. Perush Chai (tziur 36) seems to picture the Big Dipper (portion of Ursa Major) as the Agalah. What is his source? The Big Dipper does seem to resemble a carriage, and is certainly in the north.
The meaning of the Gemara when it says "Eglah in the north and Akrav in the south" is not clear. Here is what we came up with:
(a) RASHI on 56a just writes assumes that Eglah and Akrav are constellations, but in Pesachim (94a) writes that Eglah is the constellation Shor. It is clear that Rashi understands our Gemara to be referring to the two Zodiacal constellations of Eglah/Taurus and Akrav/Scorpio. Those two constellations are located on opposite sides of the ecliptic, and Rashi writes that one of them is the most southern and the other the most northern. (Not all the signs of the Zodiac are on the celestial equator since the ecliptic, on which the Zodiacal constellations are to be found, is slanted at a 23 degree angle in relation to the earth's equator).
As you pointed out, it would have been more appropriate for the Gemara to mention the constellation that comes after each of these two (that is, Te'umim/Gemini and Keshes/Sagittarius), for they are the most northerly and southerly constellations. Nevertheless, the Gemara mentions the constellation which begins to turn towards the north and the one which begins to turn towards the south of the ecliptic. Perhaps it was easier to determine this way which direction is north and which is south.
(b) RABEINU YEHONASAN M'LUNIL writes that the two constellations mentioned here "do not change their location in the summertime like the other constellations." According to his words, it may be suggested that the Gemara should read "Agalah" (and not "Eglah"), referring to the Little Dipper, whose handle is tipped by the North Star (or the big dipper, like in the Peirush Chai) which is a polar constellation that is visible in both summer and winter.
According to this, however, it is not clear what Akrav is. Perhaps Akrav refers to Hydra or Ophiucus or to one of the other northern constellations that are visible in summer as well, but which lie south of the Little Dipper. (Some of these constellations has the appearance of an Akrav as well.)
(It is unlikely to suggest that Akrav refers to Crux, which is visible near the South celestial pole summer and winter from the Southern Hemisphere.)