I use the same amount of water and sugar in my mmf all year, in TX with the air conditioner running.
The only time I have ever had problems was when I didn't have an air conditioner that worked, before I got my house. If you have central AC and it works, the humidity outside will not affect your fondant.
I'm guessing that your problem this time was that you've never made it before, and you were expecting a different texture right after making it. It will be soft and very sticky. If you make it firm, then tomorrow when you want to use it, it will be hard to knead and hard to roll out. What you need to do is wrap up the soft and sticky fondant and let it rest overnight. Don't add lots of stuff trying to get it to a workable consistency the day you make it. If it's still too soft the next day, then you need to adjust your recipe. It will be sticky the next day though...it's made out of sugar and marshmallows, of course it will be sticky. If you knead in lots of crisco, it's going to get too soft and oily to work with.
When I color mmf, I spray my hands with pam spray first, and repeat if the fondant starts sticking to my hands. I don't use crisco with fondant at all anymore. I don't like the smell of crisco, for the last six months or so it has had a really odd smell to me. So I quit using it. When I roll out fondant, I dust the work surface and the fondant with corn starch, and repeat as needed. Once I've got it rolled out big enough, I rub my hands over the fondant to remove the excess corn starch, and then keep rubbing until the fondant looks smooth and polished, before I pick it up and put it on the cake.