OK, sorry, i didn;t realize you had experience with baking, just not with ital. meringue (I was worried, actually, that you were doing a wedding cake without the experience!)
I can give you some of my own experience. I did my daughter's wedding cake a couple of years ago, in Rome, in July, it was 100 degrees, no air conditioning in the house and the place where the reception was held was outside, albeit in the evening (but it MAY have gone down to 85 by midnight). It was 3 tiers, 6 layers, chocolate butter cake (cake bible recipe) and had dark chocolate raspberry ganache between some layers (which held up very well) and white chocolate cream cheese buttercream with raspberry reduction between the other. The crumb coating was white chocolate buttercream. (It held up fine, despite the heat, and though i did;t decorate with it, i think i could have - the white chocolate really firms it up but it stays soft and fluffy). On top of that i put home made rolled fondant. Unexpectedly, due to the heat, the tiers were sliding around like crazy - had to drive a dowel through all tiers, and then put toothpicks around the edges of the layers because they were still unstable - frosted over them with royal icing borders and told the caterer to remove them before serving.
I made it during the week prior to the wedding, and put the various layers in the (small italian) freezer as i made them - my mixer is not big enough to make more than two at a time. Then i put in the filling between each of two layers, and froze those. I had to clear out the fridge (small italian fridge) on the day before the wedding so i could assemble tiers and then re-refrigerate it all together, though if i had had a big freezer, i would have frozen it instead. Then we brought it to the place the night before the wedding, where it sat in their fridge until the buffet had been served, and was brought out as people ate their meal. Stayed out two hours or so.
I'd stay away from whipped cream, unless you intend to keep it refrigerated except for a couple of hours, even if you stabilize it with gelatin. I've done that but not for something so important that has to be out and admired for a long time. Or have you tried it and it worked in the heat? You can always do some trial runs. Ganache is great and holds up even in the heat (up to a couple of hours), and would work, but if you want white you'd have to do white chocolate ganache, and as i said, i never could make it where it didn;t curdle.
I would go with the white chocolate cream cheese. It's not rich and doesn;t taste of chocolate, and you can have the bride taste it without telling her what it is.
And i would try mixing part of that with (cooled) italian meringue, which might make it fluffier. Then I'd set them both out decorated on a cardboard, in the heat, and see how tyhey hold up.
You can also always do the decorations on top of that with another frosting, like buttercream or royal icing, so tehy maintain their shape. The important thing is that the frosting of the cake is soft and not too rich, right?
And if you're really good with smoothing frosting and can get a professional touch so it looks reallyu smooth (I can't) you could just frost it with italian meringue alone, and it would hold perfectly, and you could decorate on top with buttercream (add softened butter to the meringue, or (again) white chocolate.
"Siduri said, 'Gilgamesh, where are you roaming? You will never find the eternal life that you seek...Savour your food, make each of your days a delight, ... let music and dancing fill your house, love the child who holds you by the hand and give your wife pleasure in your embrace.'"