Eggs – meringue powder

Both solutions will work.

If you are making more mering, then make sure the thickness of the meringue layer in stays the same as for the small tin. If you scale the recipe correctly this happens automatically.

I think your math is off though. The area of your 24 inch tin is only 1.77 times larger than the 18 inch pan.

Assuming your pan is round: The area of the large pan is pi*24^2 and the area of the small is pi*18^2 the ratio is therefore pi*24^2 / pi*18^2 = 24^2 / 18^2 = (24/18)^2 = 1.77 .

If your pan is square the (almost) same computation will give the same result.

The crusting is important so that any flowers or other intricate decorations will fix their shape and not slump when left out in warmer temperatures. (you don't want to refrigerate iced cakes, as when you remove them from the fridge, you'll get condensation). An icing that crusts quickly means that you can use an icing that's not quite as stiff for your decorations, and after a few hours of decorating, you'll come to appreciate that minor difference.

The crust is also a benefit when trying to get a perfectly smooth cake.

Some other recipes will crust over time, and the recipe that I got from one of my instructors uses more powdered sugar and cream of tartar, but no merangue powder. I suspect it's the starch in the powdered sugar that ensures that it still crusts up.

As for shelf life ... I'm not 100% sure. I believe that the eggwhites have been pasteurized, and I've made flowers, let them dry out, then kept them stored for months at a time ... but I can't remember if those were made with merengue powder or not. I suspect that some batches were.