Can you make the meringue ahead of time, and assemble before dinner, or does it need to be done right away?

Though, in full disclosure, I've not made these meringues, I routinely make Pavlova layers ahead of time for restaurant service, with is sort of the equivalent of what you are asking here. It depends very much on where you live. I you live in a humid climate, I'd suggest not preparing the layers no more than 12 hours in advance of serving, and in between times, securely wrapping them in plastic. As for any brittle, the rules are even more rigid. If you are in the presence of any degree of relative humidity, make and prepare the brittle as close as possible to serving time. By making a brittle, you are creating an invert sugar which will pull moisture out of the ambient air, causing it to soften, bend, and not be, well, brittle.

Meringues can be baked ahead of time. Many experienced bakers will shut the oven off, then leave the meringues inside overnight to let them firm up.

Kendra's notes (Step 6) are very specific about letting the meringues completely cool off.

Looking further at the recipe, it appears the pecan brittle is made ahead of time.

Really, the pavlova needs to be assembled and served right away so the crispy elements (meringue, pecan brittle) do not soak up too much of the pumpkin cream.

You can make the meringue and brittle the day before. If you are concerned about high humidity, I would store the meringue and brittle in airtight containers with a silica gel packet.

i think you can if you keep it in a closed container (out of fridge) and slip one of those little silica packets that come in shoes and some spices in there to control the moisture-- its all about moisture control

Absolutely! You can bake the merengue in advance, and whip up the filling and store it in the fridge until you're ready to assemble. You can also make the brittle a few days in advance.