Don't get spooked by the negative reviews. People are more likely to post a review if they have an axe to grind, and it is usually some minor issue that they just can't let go of, and so they think they'll get even with the cruise line by posting a scathing review. I don't pay attention to them. I also don't pay attention to reviews that offer nothing but soaring praise, as though the cruise was a brief trip to Shangri-La. Now, if someone posts a balanced review that has both what was good as well as what was not-so-good, that is worth reading.
Conquest sails full each cruise. If it was as bad as some reviewers claim the word would get out and people wouldn't book it.
As for the weather...nobody knows. The best advice for cruising in hurricane season is don't get your heart set on visiting all the ports. Just be flexible and realize you might not see Cozumel, but instead will see Key West.