You will put the plate and leg assembly into the tiers before you leave home. Yes, the plate will be sticking up in the air. Then when you get onsite, you will pick up each cake on it's cardboard and place it on the plate, and add your border.
Uhhh . . . what do you mean "the cakes are already iced on the plates." The SPS plates? No, that's not how you use this system. The cakes stay on their cardboards.
How do you propose to attach the legs to the plate and push the assembly into the tier?