Do a taste test of the both to make sure you like it/them. For tall cakes I use packing crates if they're stacked or put each tier in a cakebox and assemble on site. But notice how on t.v shows they hardly ever use boxes ?
Check out http://cakecentral.com/gallery/34174/christening-cake. I am thinking a two-tier cake, maybe a 16" and a 12", with the 12'' being a replica of the topper here. You should be able to work in quite a few decorating genres. You can do a baby (F.I No.5 is good) and an angel if you're unsure about crosses and chalices.
The height of the cake would determine the process used for me. But for something like that I would do like Bakencake said but I'd do the base first and then the dome. What I did a couple times was measure EXACTLY and then thin/bevel the edge so thet when it came time for the seam, it was barely noticeable.
You SIMPLY MUST wait till the cake is thawed before applying fondant. That it's already iced is a huge bonus. Applying fondant to cold cake causes condensation and you get a sticky, icky possibly runny fondant.
Isomalt for sure. Do you have a SASSIE? Having one makes life with isomalt way easier, but if you don't just pour the boiled isomalt into a glass cup with a pouring lip and fill up your molds. There's a recipe for isomalt gems on [url]sugarcraft.com.[/url]