How are the legs attached? Are they screwed onto the frame??
Yes they are screwed into the stand and tightened with a small wrench.
As long as there isn't a weight limit, I would think you'd be ok using the long legs. I would probably remove the top tier before serving, pushing down to cut the cake might put a lot of strain on those legs.
Put the long legs on and then put some books with the equivalent weight on it and test how sturdy it is.