Also if you're using two cake mixes and fill the pans all the way up to the top, that is too much. If you're doing two mixes you should have plenty left over for cupcakes. Fill them 3/4 full and bake them at 350 with the strips.
Is there any way you could just set it on the cake the day of the wedding? That way you wouldn't have to worry about it. If it is heavy, you could do as you said and cut a board and put that under the fullest part of the arrangement, and then just put the dowels under where the board will go.
There was at least one lady who shared her recipe with me, and she used all shortening. It was two different types of shortening. I never did make it because I prefer butter. I think if it works for you and you were pleased, then go with it. I think if you have to use shortening, high ratio is the better bet because it doesn't have the greasy texture and taste of Crisco.
The Magic Line pans are awesome. Why did you use three heating cores for an 8" cake? I don't use heating cores even in 12" pans.
You mentioned you used two box mixes. Were they doctored mixes (adding sour cream and other ingredients not called for on the box), or did you just mix them according to the package directions?
I use the baking strips and sometimes I bake at 325, but most of the time at 350. How long did you leave the cake in the oven?